Because 4 and 8 are the only numbers 1-10 that are divisible by four.
This would give Amy a 2/10 chance of rolling a number divisible by 4.
To turn 2/10 into a percent, the denominator has to be 100 so if you multiply 10 by 10 you will get 100, you will also have to multiply 2 by 10 to get 20 and you will get 20/100