The Professor's Response

Prof. I am having same issue on #40 ch 3 Probability

It says find 10C2 on calculator answer is 45. But if I attempt to work it out this is what I get:
1st try: (10/2)= 10!/2! Then I get 10*9*8*7*6*5*4*3*2!/2! I cancel both 2!'s so am left with 10*9*8*7*6*5*4*3=1,814,400. Can you explain please. I know I got right answer using calculator but this simplifying is driving me crazy! Not sure what am doing wrong.

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Professor McGukian

Hi Natalia,

You need to be a precise record keeper if you are going to perform the calculation by hand. In this case, you did not fill in the formula properly before starting the work. You left out your 8!. Use your formula card or commit the formula to memory. 

10C2 = 10!/(2!*8!) = 10*9*8!/(2*1*8!) = 10*9/2 = 45


Professor McGuckian 

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