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1) A die is rolled 3 times, let x be the sum of the 3 results .

what is var(x) ?

I started by calculating E(x)= Segma(1 to 3 (E(Xi)) when Xi is the number we got in every throw .

E(Xi)=3.5

then E(x) = 10.5 (Expected of the sum)

now what should I do to find the var of x ?

2) we have n balls and n^2 chains , we throw the balls randomly into the chains.

we pick y randomly - a chain , what's the probability that it has exactly 3 balls in it .

ok, I've kinda solve this question but i'm not sure if the answer is correct .. we choose 3 balls from the n balls , and we do it in n!/((n-3)!*3!) and we calculate every ball by the probability that it goes into y , 1/n^2 .. thus final answer is :

n!

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(n-3)!*3!*n^6

can you help me complete the first Q and check the second one for me.

thanks.