Sorry if this is too easy....

A, B, and C, have $10 each. They call D to buy snacks for them. The snacks cost them $25. A, B, and C each keeps 1$ of the change and give $2 to D.

Re-sum the money : A, B, and C spent $9 each and D got $2, so total money = 3 x $9 + $2 = $29

Well, the sum supposes to be $30, doesn't it? Where is the missing $1 ?