Question

I toss 3 fair coins, and then re-toss all the ones that come up tails. Let X denote the number of coins that come up heads on the first toss, and let Y denote the number of re-tossed coins that come up heads on the second toss. (Hence 0 ≤ X ≤ 3 and 0 ≤ Y ≤ 3 − X.)

(a) Determine the joint pmf of X and Y , and use it to calculate E(X + Y ).

(b) Derive a formula for E(Y |X) and use it to compute E(X + Y ) as E(E(X + Y |X)).

Answer 1