This is actually part of a statistics problem I'm working but I forgot how to do this and if someone can refresh my memory, I might be able to solve these.

Question

The problem is with factorials.

20! / 5! (20-5)!

I know I can reduce and cancel, but how and where?

If I do
20! / 5! (15)! I get 20! / 75!
Here's where I think I'm screwing up. I think I can cancel the 15! on the bottom and 15! from the top, but then I'm left with 20*19*18*17*16=1860480.
Calculate out the 5! on the bottom, I get 120. Divide the two and I get 15504. Is this correct? Is there an easier way?

Answer 1

20!/[5!*15!) = 20*19*18*17*16/(1*2*3*4*5)
= 20*19*18*17*16/120
= 19*18*17*16/6
= 19*3*17*16 = 15504

Your answer is correct. The only easier way is to use a calculator.

Answer 2

the mass of a neutron is 1.67(10^(-27))kg. what is the mass of 7.03(10^(21))neutrons?