Deven has four ducks labeled A, B, C, and D. How many ways are there for Deven to arrange his ducks in a row if duck A must be before duck D, and duck D must be beofre duck B?
+600 We have ADB and C can go anywhere. C can be before the A, in the middle of A&D, in the middle of D&B, after B. That's $4$ ways.
