Challenge of the Week

Sunday 13th Sept 2025

"I forgot to factor that in..."

Definition: The trailling zeroes of a number are the numbers at the end of it. E.g. 40200 has two trailling zeroes.

None of the following should be attempted by calculating the value of the factorial! That would be bonkers!

1) Find how many trailling zeroes 10! has.

2) Find how many trailling zeroes 20! has.

3) Find how many trailling zeroes 100! has.

4) Explore larger factorials. Can you find a way to explain how many zeroes n! has?