algebra

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.

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?

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asked by Dawn
  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.

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

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    posted by antonio

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