60 Fun Facts About 4th Grade Math

Which of these numbers is a prime number?

7 is prime: nothing divides it but 1 and 7, and huge primes keep data safe online, because multiplying two of them is easy while working out which two were multiplied is very hard.

What do we call a whole number with more than two factors, like 8 or 15?

Composite — every such number is made by multiplying primes together, as 8 is 2 × 2 × 2, which is why primes get called the building blocks of all numbers.

What do we call the distance all the way around the outside of a shape?

Perimeter is found by adding up the length of every side, which is how you would work out how much fence a garden needs.

What is the area of a rectangle 6 cm long and 4 cm wide?

24 cm²: picture 4 rows of 6 one-centimetre squares, which is why area is counted in square units.

What is the distance all the way around a rectangle 5 m long and 3 m wide?

16 m: add the length and the width, then double the total, because a rectangle's opposite sides always match.

In the fraction 3/4, what is the number on top called?

The numerator: a fraction is also a division waiting to happen, so 3/4 means 3 divided by 4, which comes to 0.75.

Which fraction is greater than 1?

7/4: a top number bigger than the bottom earns the name improper fraction, though nothing is wrong with one, and this is just 1 3/4 in disguise.

How many degrees are in a right angle?

90: to mark one without any tool, measure out a triangle with sides of 3, 4 and 5, a trick surveyors still use.

An angle bigger than a right angle but smaller than a straight angle is called what?

Obtuse: the word comes from the Latin for blunt, which is also why a person slow to catch on gets called obtuse.

How many degrees make a full turn all the way around a point?

360: the number splits evenly by every whole number from 1 to 10 except 7, which makes a circle easy to cut into equal parts.

Which tool measures the size of an angle?

A protractor: most carry two rows of numbers running in opposite directions, so an angle can be read whichever way it opens.

What do the three angles inside a flat triangle add up to?

180°: that only holds on a flat surface, because a triangle drawn on a globe has angles that add up to more.

A triangle with three sides all the same length is called what?

Equilateral: it is the very first shape Euclid's geometry book builds, from two overlapping circles whose crossing point becomes the third corner.

A triangle with exactly two sides the same length is called what?

Isosceles: the angles facing its two equal sides always match too, a proof nicknamed the bridge of asses, perhaps because weaker students could not get past it.

What is any shape with four straight sides called?

Quadrilateral: the family takes in squares, rectangles and kites, and every square also counts as a rectangle, though not the other way round.

What shape are the cells in a honeycomb?

Hexagon: six-sided cells fit together with no gaps using the least wall for the space they hold, a hunch about bees that was only proved in 1999.

What shape is a standard stop sign?

Octagon: in the 1920s American road engineers gave each kind of sign its own outline, so a driver could tell a stop from any other warning by shape alone.

A five-sided shape is called what?

A pentagon: the Pentagon near Washington got its outline from the roads around the first plot picked for it, and the five-sided plan stuck even after the site moved.

A closed flat shape made only of straight sides is called what?

A polygon: Archimedes pinned down the value of pi by trapping a circle between two 96-sided ones, one drawn just inside it and one just outside.

The distance from the centre of a circle to its edge is called what?

The radius: it is the gap a compass holds fixed between its spike and its pencil, which is why the curve it draws comes out perfectly round.

If a circle's radius is 5 cm, how wide is it all the way across through the middle?

10 cm: that is two radii end to end, and a string wrapped once around the edge would stretch just over three of those widths, the number called pi.

Two lines that stay the same distance apart and never cross are called what?

Parallel: railway rails are the classic pair, yet they seem to meet far away, a trick of the eye that artists copy with a vanishing point.

Two lines that cross to make right angles are called what?

Perpendicular: builders check that a wall stands perpendicular to level ground with a plumb line, a weight on a string that gravity pulls straight down.

A straight path that stops at two endpoints is called what?

A line segment: in geometry a true line never ends in either direction, so everything anyone actually draws with a ruler is only a segment of one.

A fold that makes both halves of a shape match exactly is called what?

A line of symmetry: stand a small mirror along it and the reflection rebuilds the whole shape, which is why it is also called a mirror line.

Which solid has six square faces?

Cube: that sameness is why dice are cubes, since six matching faces give every number an equal chance of landing on top.

What is the answer to a multiplication problem called?

The product: swap the two numbers and it stays the same, so 3 × 8 and 8 × 3 both come to 24, while 8 ÷ 2 and 2 ÷ 8 give different answers.

In 30 ÷ 2 = 15, which number is the quotient?

15: the quotient is the answer to a division, and multiplying it back checks the work, since 15 × 2 returns the 30 you started with.

When you divide 17 by 5, what is the remainder?

2: three fives make 15, leaving 2 over, and a remainder can never reach the number you divide by, or one more group would fit.

Which of these is a factor of 12?

6: twelve splits evenly among 2, 3, 4 or 6 people, a handiness that ten, with only 2 and 5, cannot match.

Which of these is a multiple of 7?

42: it is 7 × 6, whereas a number that merely ends in 7, such as 17 or 187, need not be in the seven times table at all.

What is the value of the 7 in 4,725?

700: each step to the left makes a digit worth ten times more, so the same 7 one place further over would count for 7,000.

9 is a square number because it equals what?

3 × 3: nine dots can be laid out as a grid three across and three down, which is exactly where square numbers get their name.

Which multiplication means the same as 5 + 5 + 5?

3 × 5: count by fives three times, 5, 10, 15, and you have done the multiplication, which is why skip counting is taught as the way into the times tables.

In Roman numerals, what number does IX stand for?

9: a smaller numeral placed before a larger one is subtracted, yet many clock faces ignore that rule and show four as IIII.

In Roman numerals, which letter stands for 100?

C: the sign probably did not begin as that letter at all, because an older symbol for a hundred seems to have been reshaped over time until it matched the Latin C.

How many millimetres are in a centimetre?

10: metric prefixes all step by powers of ten and attach to any unit, so the same steps work for litres and grams as for lengths.

Which unit of length equals 12 inches?

A foot: for decades the United States actually had two of them, because surveyors kept an older, very slightly different foot after the unit was redefined.

How many feet are in a yard?

3: but a square yard holds nine square feet, not three, because the three counts along the length and again along the width.

How many ounces are in a pound?

16: that is the everyday pound, but in the troy system used for gold and silver a pound holds only 12 ounces.

How many quarts are in a gallon?

4: a quart is a quarter of a gallon on both sides of the Atlantic, but the British gallon is about a fifth bigger than the American one.

How many seconds are in one hour?

3,600: 60 × 60, and the sixties go back to the base-60 counting of ancient Sumer and Babylon, which astronomers carried into the way we split time.

In a leap year, which month gets an extra day?

February: the extra day keeps the calendar in step with the seasons, yet century years such as 1900 skip it unless they divide by 400.

How many years are in a century?

100: counted strictly from AD 1, each century starts in a year ending in 01, so the 21st began on 1 January 2001, though popular usage started it a year sooner.

How many items are in a dozen?

12: a baker's dozen is 13, said to come from bakers slipping in a spare roll so that nobody could punish them for selling a batch that weighed light.

What is a 1 followed by six zeros called?

One million: three zeros further the names once disagreed, because a British billion long meant a million million rather than a thousand million.

Rounding means changing a number to what kind of value?

A close, simpler one: on an exact tie, Python and many other computer languages round to the even neighbour, so 0.5 becomes 0 while 1.5 becomes 2.

Which fraction is the same amount as one half?

2/4: multiplying the top and bottom of a fraction by the same number changes how it looks but not its size, so 1/2, 2/4 and 50/100 all match.

In a number like 25.97, what is the dot between the 5 and the 9 called?

A decimal point: much of Europe writes a comma in its place, and the world's measurement body officially accepts either mark.

How many grams are in one kilogram?

1,000 g: until 2019 the kilogram was not a formula but an object, one metal weight in a vault near Paris that every other kilogram was checked against.

About how much does one litre of water weigh?

1 kg: that is no accident, because the kilogram was originally defined as the mass of a litre of water, and the match still holds almost exactly.

How many years are in a millennium?

1,000: as the last one turned, programmers feared that computers storing years as two digits would take the new year for 1900, the Y2K bug.

What are the corners of a cube called?

Vertices: add them to the faces, subtract the edges, and any box, pyramid or prism gives exactly 2, a pattern named after Euler.

Which solid has two flat circular faces and one curved side?

Cylinder: unroll the curved side and it flattens into a plain rectangle, so the whole solid can be made from two circles and one rectangle of paper.

What do you call a triangle that has one square corner?

A right triangle: draw any triangle inside a half circle with the straight edge as one side, and the corner that touches the curve always comes out square.

What is any four-sided shape with four equal sides called, whatever its angles?

A rhombus: every square is one, since a square is simply a rhombus with square corners, and the leaning diamond on a playing card is another.

At what temperature does water boil on the Fahrenheit scale?

212 °F: only at sea level, though, because the boiling point falls as air pressure drops, so water on a high mountain boils at a lower temperature.

In Roman numerals, what number does MM stand for?

2,000: the earliest Romans would not have recognised it, because they wrote a thousand with a different symbol and the letter M only took over later.

Multiplying any number by 1 gives what?

The same value: adding has a do-nothing number too, which is zero, yet multiply by zero and any number is wiped out instead of kept.

Which digit of a whole number tells you whether it is even?

The last digit: if it is 0, 2, 4, 6 or 8 the number is even, and the list starts at 0 because zero itself is an even number, however odd that sounds.