[Xylric]

It's a trick question. There isn't any missing dollar.

3x 10 = 30. (They each chip in $10).

30-25 = 5 (The pizza was actually $25)

1+1+1+2 = 5 (The deliveryman keeps $2 as a tip, gives each of the three back $1)

25 + 5 = 30 (Self explanatory)