Prime Digits

Number Theory Level 2

You have a list of numbers from 1 to 999. If you cross out the numbers with prime digits (2, 3, 5, and 7), how many digits will be left?

1 solution

Timothy Wong
Apr 26, 2014

First, you remove numbers 200-299, 300-399, 500-599, and 700-799 ( $-400$ numbers)

Secondly, you remove numbers 20-29, 30-39, 50-59, 70-79 ( $-40 \times 6=-240$ numbers)

Lastly, you remove numbers 2, 3, 5, and 7 ( $-4 \times 6 \times 6=-144$ numbers)

$999-400-240-144=215$ numbers.