The multiplication factor is 5/4 i.e. when you multiply 100 by 5/4, you get 100*5/4 = 125.

What is the multiplying factor?

: the ratio of the number of neutrons produced in a nuclear pile to the number disappearing that must equal or exceed unity for a chain reaction to take place. — called also reproduction constant, reproduction factor.

What does multiplying by 1.25 do?

Remember that 100% of a number is the number – so, the same as multiplying by 1. Whenever you need to find a % of a number, multiply it by the % AFTER you move the decimal place over two spaces. So, to find 125%, multiply by 1.25. To find 225%, multiply by 2.25, etc.