Question

How many times does the digit `4` occur in the page numbers of a `50` page book?

Answer 1

`15`

Explanation:

The digit `4` can occur in the units or tens positions.

It occurs in the units position in the numbers:

`4, 14, 24, 34, 44`

It occurs in the tens position in the numbers:

`40, 41, 42, 43, 44, 45, 46, 47, 48, 49`

So there are a total of `5+10 = 15` occurrences of the digit `4`.

If the question asked in how many page numbers the digit `4` occurs then the answer would be `14` - avoiding counting `44` twice. However, since it asks for the number of occurrences of the digit `4`, `44` should be counted twice.