New Scientist Enigma 511 – Double, Double

by Susan Denham

From Issue 1663, 6th May 1989

I wrote an odd number on the board and asked the class how many numbers (including the original number itself) could be made by writing exactly the same digits but in different orders. For example, if the number had been 5051, the answer would have been nine, namely 5051, 5015, 5105, 5150, 5501,5510, 1055, 505 and 1550). Clever Dick got the right answer immediately, so to keep him busy I told him to repeat the exercise with exactly double my original number.

“That just doubles the number of ways, Miss,” he reported. I told him to double again and repeat the exercise, and again he reported “that doubles the number of ways get again, Miss.”

So I told him to double the number yet again and to repeat the exercise with the four-figure answer. “It’s doubled the number of ways again, Miss,” he replied and, as always, he was quite right.

What number did l write on the board?