Did you solve it? Do you have a snout for numbers?

By Alex Bellos • June 8, 2026 • Science

The answer to today’s puzzle

Earlier today I set this elegant number puzzle. Here it is again with a solution. Nose to tail There is a number N beginning with 4 such that moving the 4 to the end of it creates a new number that is a quarter of N. In other words N is of the form 4[…], where […] is a sequence of digits, and N ÷ 4 = […]4 What is the lowest possible value of N? Solution My strategy is to start by trying to find a two-digit N, and gradually increase the number of digits until we’re done. Two -digits. Say N = 4[?], where [?] is a digit. The only possible value for [?] is 1 because we know that a quarter of 4[?] is [?]4 and a quarter of 4 is 1. But 14 is not a quarter of 41, so we conclude N has more than two digits. Three digits. Let N = 4[??]. For the same reason as before, the second digit of N must be 1. So N = 41[?]. We know that a quarter of 41[?] = 1[?]4, which is the same as saying that 4 x 1[?]4 = 41[?]. We deduce that the final digit of N must be 6, since 4 x 4 is 16. However, a quarter of 416 is not 164, so N has more than three digits. Four digits Let N = 4[???]. For the same reasons as above, N = 41[?]6. We know that a quarter of 41[?]6 = 1[?]64, which is the same as saying that 4 x 1[?]64 = 41[?]6. We deduce that the penultimate digit of N must be 5, since 4 x 64 = 256. However, a quarter of 4156 is not 1564, so onwards we go. Five digits N = 41[?]56 We know that 4 x 1[?]564 = 41[?]56. Since 4 x 564 = 2256, the antepenultimate digit of N must be 2. But a quarter of 41256 is not 12564, so we need to carry on. Six digits N = 41[?]256 We know that 4 x 1[?]2564 = 41[?]256. Since 4 x 2564 = 10256, we know that the [?] must be 0. This works! We have our answer: N = 410256 = 4 x 102564 I hope you enjoyed this puzzle. I’ll be back in two weeks. Source: Moscow Mathematical Olympiad 1983, via @mathematicsproblems and Kevin Gately I’ve been setting a puzzle here on alternate Mondays since 2015. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.

Source: The Guardian