40 Fun Facts About Subtraction

The Wright brothers first flew in 1903 and Apollo 11 reached the Moon in 1969: how many years apart?

Just 66 years separate the two, and Neil Armstrong carried scraps of fabric and wood from the Wright Flyer down to the lunar surface.

A modern piano has 88 keys, 52 of them white: how many black keys does that leave?

The 36 black keys complete the standard 88, but the Imperial Bösendorfer runs to 97 keys, its extras sometimes hidden under a small hinged lid.

On the Fahrenheit scale water freezes at 32 degrees and boils at 212: how many degrees separate them?

The gap is 180 degrees, so one Fahrenheit degree is 1/180 of the span. Celsius splits the same span into only 100 steps, which makes each of its degrees bigger.

Of the Seven Wonders of the Ancient World, only the Great Pyramid of Giza survives: how many have been lost?

Six have been lost, and the survivor was already the oldest by far. The Great Pyramid kept its white stone facing until around AD 1300, when locals took it for building material.

The Hundred Years' War between England and France began in 1337 and ended in 1453: how many years was that?

It lasted 116 years, longer than its name admits, though the fighting was broken up by long truces and is usually divided into three phases.

Opposite faces of a standard cubic die always add up to seven: which number is opposite the 5?

The 2 sits opposite the 5, a rule so old that a terracotta die dug up in the Indus Valley city of Mohenjo-daro, made before 1900 BCE, already follows it.

Disney's 101 Dalmatians include 99 puppies, 15 of them Pongo and Perdita's own: how many did the pair adopt?

They adopt 84, all bought from shops for Cruella de Vil's planned fur coats. With the two parents, that brings the household to the 101 of the title.

To switch calendars in 1752, Britain cut 11 days from the 30 in September: how many days did that month keep?

September 1752 kept only 19 days: Wednesday the 2nd was followed by Thursday the 14th. Tales of riots over the lost days are very likely a myth, with no record of any from the time.

Abraham Lincoln's Gettysburg Address of 1863 looked back 87 years to the nation's founding: which year did he mean?

He meant 1776, the year of the Declaration of Independence. Lincoln spoke for only about two minutes, after a two-hour oration by Edward Everett.

The Titanic sank in 1912 and Robert Ballard's team found the wreck in 1985: how many years had passed?

It lay hidden for 73 years. The search was made possible in part by a US Navy job: Ballard's team first had to map the wrecks of two sunken nuclear submarines.

Children grow 20 milk teeth and adults have 32 permanent ones: how many more teeth does a full adult set have?

A full adult set has 12 more. The last to arrive are the third molars, better known as wisdom teeth, which usually hold off until the late teens or later.

Adding two numbers gives a sum: what is the result of subtracting one number from another called?

The result is the difference. Accountants can ask for one without any sign at all: a lower figure written in red usually means it is to be subtracted from the figure above.

A standard deck has 52 playing cards, 13 of them hearts: how many cards remain when every heart is taken out?

Take out the hearts and 39 cards remain. One of the cards removed is a 'one-eyed jack': the jack of hearts is drawn in profile, as is the jack of spades.

On a standard number line with zero in the middle, subtracting a positive number moves you in which direction?

You move left, toward the smaller numbers. Keep stepping past zero and the line carries on into the negatives, which is why taking a bigger number from a smaller one still has an answer.

A ten-pin bowler knocks down 7 pins with the first ball of a frame: how many must fall next for a spare?

The last 3 pins must fall for a spare, which clears the whole rack with the second ball. Clearing every pin with the first ball is a strike.

When a column's top digit is too small to subtract from, American schools teach borrowing, also known by which name?

Borrowing is also called regrouping, and pupils track it with small pencil marks known as crutches, which spread through American classrooms after a study by William Brownell.

Hawaii became the 50th US state in 1959: how many states had joined the Union after the original 13?

Congress has admitted 37 states since the original 13. Texas had a long wait: it asked to join in 1837, but fears of conflict with Mexico held up its admission for years.

The Solar System has eight planets, of which Mercury, Venus, Earth and Mars are rocky: how many are giant planets?

The other 4 are giants, but not all alike: astronomers now call Jupiter and Saturn gas giants and class Uranus and Neptune as ice giants.

Elizabeth II came to the British throne in 1952 and died in 2022: how many years did she reign?

She reigned for 70 years, the longest of any British monarch. In 2015 she passed the mark set by her great-great-grandmother, Queen Victoria.

In Roman numerals a smaller symbol written before a larger one is subtracted: which number does XC stand for?

XC is 90, or ten taken from a hundred. Ancient Rome was not strict about the rule: the numbered gates of the Colosseum write IIII where modern usage has IV.

In statistics, subtracting the smallest value in a data set from the largest gives which simple measure of spread?

That gap is the range, and it comes out in the same units as the data: the range of a class's heights is itself a height.

The word minus comes straight from Latin, where its opposite was plus: what does minus mean in Latin?

Minus is Latin for 'less', just as plus means 'more'. Luca Pacioli's 1494 textbook wrote the pair as a p and an m with a bar on top, short for the Italian più and meno.

Pluto was discovered in 1930 and stripped of full planet status in 2006: how many years did it count as a planet?

Pluto was a planet for 76 years. Its demotion made such a stir that the American Dialect Society voted 'plutoed' its word of the year.

A darts player on 501 hits treble 20, worth 60 points, with the first dart: what score remains?

The scoreboard drops to 441. Darts is subtraction as sport: each turn's points come off the total, and the winning dart must land in a double or the bullseye.

Dubai's Burj Khalifa stands 828 metres tall and the Eiffel Tower 330 metres: how much taller is the Burj Khalifa?

The Burj Khalifa is 498 metres taller, yet the Eiffel Tower was itself the tallest structure in the world from its opening in 1889 until 1930.

Which word names the weight of an empty container, which a scale subtracts from the gross weight to show the net?

It is the tare, a word that reached English through Italian from an Arabic term for something thrown away. Kitchen scales have a button that resets the display to zero for the same job.

Swapping the two numbers in a subtraction changes the result, so the operation lacks which property that addition has?

Subtraction is not commutative: reversing the numbers flips the sign of the result, so mathematicians describe the operation as anti-commutative instead.

Mount Everest is 8,849 metres high and K2, the second-highest mountain, 8,611 metres: how much higher is Everest?

Everest is 238 metres higher, but K2 has the fiercer name: it became the Savage Mountain after a climber on a 1953 American expedition said it tries to kill you.

Counting up from 567 to 1,234 in easy hops of 3, 30, 400 and 234 shows that 1,234 − 567 equals what?

The answer is 667. The 'same change' trick gets there too: nudge both numbers up by 3 and then by 30, and the sum becomes the far easier 1,267 − 600.

Which French thinker's 1640s calculating machine had gears that turned only one way, making subtraction awkward?

Blaise Pascal got round it by entering a number's complement, which turns a subtraction into an addition. He built the machine to lighten the sums of his father, a tax official.

In the subtraction 50 − 20 = 30, what is the 50, the number being taken away from, called?

It is the minuend, a word built on the Latin minuere, 'to reduce or diminish', so it literally means the 'thing to be diminished'.

Mathematicians class which game, played by removing matchsticks or coins from piles, as a notable subtraction game?

Nim fits because every move removes tokens from a single pile, and it can be played perfectly: a well-known strategy is built on a quantity called the nim-sum.

With no year zero, 1 BC runs straight into AD 1: how many years separate 5 BC from AD 5?

Only 9 years pass, not ten, because the count skips zero. Astronomers find that so awkward that their own year numbering puts a year zero where 1 BC would be.

Which symbol found on most calculator keypads was used by earlier writers as a sign for subtraction?

The ÷ sign stood for subtraction before Johann Rahn's 1659 algebra book gave it to division, and a Norwegian tax form was still using it as a minus in 2010.

Subtracting 1,467 from 7,641 gives which number, famous as the constant of Indian mathematician D. R. Kaprekar?

It is 6,174, built from the same digits you started with. Almost any four-digit number, sorted high and low and subtracted over and over, ends up there within seven steps.

Mount Whitney rises 14,505 feet above sea level and Badwater Basin lies 282 feet below it: what is the vertical gap?

The gap is 14,787 feet, because taking away a depth below sea level means adding it on. Both are in California: the highest point of the contiguous United States and the lowest in North America.

At Loma, Montana, in 1972 the temperature rose from −54 °F to 49 °F within a day: by how many degrees?

The rise was 103 degrees, because climbing out of a negative means adding the two figures. A warm Chinook wind drove the swing, the official US record for a 24-hour temperature change.

Which Indian mathematician's treatise of 628 laid down rules for subtraction with zero and with negative numbers?

Brahmagupta ruled that a negative minus zero stays negative, in a treatise regarded as the earliest known to treat zero as a number in its own right and not just a placeholder.

An English-language Scrabble set has 100 tiles, two of them blank: how many tiles carry a letter?

The other 98 tiles each carry a letter and a point value from 1 to 10. A blank works as a wild card that can stand for any letter, but it scores nothing.

The Berlin Wall went up in 1961 and was opened in 1989: for how many years did it divide the city?

It divided Berlin for 28 years. Once it opened, crowds climbed on top and souvenir hunters spent weeks chipping parts of it away.