[Add maths Form 5] Permutations

I was thinking the answer would be 3x2x10x10x10x10 but according to the answer sheet it's (3x2x10x10x10x10)-1. Can someone explain why?

I was thinking the answer would be 3x2x10x10x10x10 but according to the answer sheet it's (3x2x10x10x10x10)-1. Can someone explain why?