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Two numbers are chosen at random from 1, 2, 3, ..., 20.  Find the proability that their product is divisible by 4.

 May 3, 2020
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I will take a crack at this one:

 

First, you have to multiply all 20 numbers by each other in which case you will have: 19 x 20/2 =190 products

 

Second, you have to take the 190 products mod 4 to see how many of them are divisible by 4. I did that for

 

you and counted 96 such numbers. Therefore, the probability is: 96 / 190 =48 / 95

 May 3, 2020

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