Solution: D
We note that if an adult crosses the river, there has to be a child on the other side to bring the raft back, otherwise the adult has to bring the raft back and two journeys have been wasted.

So the first journey must involve two of the children crossing, and one of them bringing the raft back. Then an adult can cross the river, and the child on the other bank can bring the raft back. In this way it has taken 4 journeys to get one adult across the river, with the other adult and the two children in their original position. With another 4 similar journeys, the second adult can cross the river. This leaves the two children in their original position. They can now cross the river in one journey. In this way it takes 9 journeys to get everyone across the river.
24. Two adults and two children wish to cross a river. They make a raft but it will carry only the weight of one adult or two children. What is the minimum number of times the raft must cross the river to get all four people to the other side? (N.B. The raft may not cross the river without at least one person on board.)
A 3
B 5
C 7
D 9
E 11