Did you know?
In 628 CE, Indian mathematician Brahmagupta wrote rules for adding, subtracting and multiplying with zero. His book is the earliest known text to treat zero as a number in its own right.
When a friend said taxi number 1729 seemed dull, Srinivasa Ramanujan disagreed: it is the smallest number that is the sum of two cubes in two different ways! (A cube is a number multiplied by itself three times.)
D. R. Kaprekar was a schoolteacher in Devlali, Maharashtra, who loved playing with numbers. He discovered the surprising magic of the number 6174. Try it below!
Try this: Chase the magic number
Pick a four-digit number with at least two different digits, like 3524. Arrange its digits from biggest to smallest (5432) and smallest to biggest (2345), then subtract: 5432 − 2345 = 3087. Repeat with your answer, keeping any zeros. Within seven rounds you will reach a number that just repeats itself. Try other starting numbers too!
Show answer
6174, called Kaprekar's constant
Go further: The pairing trick
How fast can you add 1 + 2 + 3 + … + 100? Try pairing the numbers: 1 + 100 = 101, 2 + 99 = 101, 3 + 98 = 101, and so on. There are 50 such pairs, so the total is 50 × 101 = 5,050. No long adding needed! This trick works for any list of numbers that go up by the same amount each time.