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Here is what I did.

First let u be the velocit of the boat

After the first shoes:

(M + m)u = mv

u = mv(1/(M +m))

After the second shoes:

Change in momentum of the boat = Change in momentum of the shoe

So, Mu'- (M + m)u = mv, but (M + m)u = mv

Therefore Mu' = 2mv and u' = 2mv/M

u + u' = mv(2/M + 1/(M + m)) is the final velocity of the boat.

But the correct answer is mv(1/(M + m) + 1/(M + 2m)), can anyone please tell me where I've got it wrong?