Explanation:
One number is divisible by another if, after division, the remainder is 0.
For example, 12 is divisible by 3, because 12/3 = 4 and the remainder is 0. But 11 isn't divisible by 3 because 11/3 = 3 and the remainder is 2.
So, here, our number is 45^10 • 5^40 and we need to determine whether it is divisible by 25^20.
25^20 can be written as (5^2)^20 which equals to 5^40.
So, basically, we have:
45^10 • 5^40 / 5^40
When we cancel out the same numbers, we are left only with 45^10 and no remainders, meaning these two numbers are divisible.