1
The only number with its letters in reverse alphabetical order.
6
A perfect number. The numbers that go into six – 1, 2 and 3 – add up to six.
11½
The number of days it would take – counting at one digit per second – to count to one million.
13
The number in a baker’s dozen. This is why in the 13th century, there was a law called the Assize of Bread and Ale in which it was decreed that bakers who short-changed their customers faced serious punishment – like having an arm chopped off. So, rather than risk such a fate, bakers would routinely hand over thirteen items when asked for a dozen – just in case they’d miscounted or one of the items was defective in some way.
13
The smallest number with eight letters.
The smallest number with seven letters.
23
The number of people who need to be in the same room to make it probable that two of them will share the same birthday. But although there only have to be 23 people present to make it probable that any two share the same birthday, if you specify the date of that birthday they share – say 11 February – then there have to be 253 people present to make it probable.
28
The calendar repeats itself every 28 years.
37
The numbers 111, 222, 333, 444, 555, 666, 777, 888 and 999 are all multiples of 37.
40
The only number with its letters in alphabetical order.
52
The number of letters in Acetwothreefourfivesixseveneightninetenjackqueenking – the same as the number of cards in the deck.
153
A number which is the ‘sum of the cubes of its digits’ (i.e.153 = 1+125+27).
293
The number of different ways there are to make change for a dollar.
The total number of gifts that ‘my true love gave to me’ in the carol ‘The Twelve Days of Christmas’.
600
A ‘centillion’ is the number 1 followed by 600 zeros (300 in the US).
1,000
This number contains the letter A (none of the words from one to nine hundred and ninety-nine does).
1,089
If you multiply 1,089 by 9 you get 9,801. It’s reversed itself. This also works with 10,989 or 1,099,989 and so on.
1666
The Roman numerals for this are MDCLXVI (1000+500+100+50+10+5+1) and the year is famous as being the one time in history when the date is written with all of the Roman numerals from the highest value to the lowest value.
2,520
The number 2,520 can be divided precisely by 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10.
5,050
The total when you add up the numbers 1 to 100 consecutively (1 + 2 + 3 + 4 + 5, etc.).
10,000
The ancient Greeks had no way to describe any numbers more than 10,000 and so simply called them a myriad.
21,978
If you multiply 21978 by 4, you get 87912 – which is the digits 21978 in reverse order.
37,037
Multiply 37,037 by any single number (1–9), then multiply that number by 3. Every digit in the answer will be the same as that first single number.
115,200
The number of possible permutations of six couples at a twelve-person table so that men and women alternate and are also not sitting next to their partners.
142,857
When 142,857 is multiplied by any number from one to six, the result is a number containing the same digits in the same order – but starting in a different place.
1 x 142,857 = 142,857
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2 x 142,857 = 285,714
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3 x 142,857 = 428,571
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4 x 142,857 = 571,428
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5 x 142,857 = 714,285
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6 x 142,857 = 857,142
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When you multiply 142,857 by 7: it equals 999,999
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When 1 is divided by 7 it comes to 0.142857,142857,142857
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If you multiply 142,857 by 8, you get 1142856. If you take off the first digit (1) and add it to the remaining six digits, see what happens: 1 + 142856 = 142857
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Similarly, if you multiply 142,857 by – say – 17, you get 2,428,569. Now take off the first digit (2) and add it to the remaining six digits and see what happens: 2 + 428,569 = 428,571 – which is, of course, the original number in stage three of its cycle.
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You can do this with every number except multiples of seven (although these also produce interesting results).
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If you take the number 142,857 and split it into two – 142 and 857 – and then add those two numbers, you get 999.
999,999
The 762nd to 767th digits of pi are 999999.
1,274,953,680
The number that uses all the digits 0–9 that can also be divided by any number from 1 to 16 without leaving a remainder.
31,557,600
The number of seconds in a year.
