Independent Pursuits: Puzzle

BE CAREFUL about this one. Not only do most people get it wrong; even when told the right answer, they cannot see how it is done.

Four men have to cross a river. There is only one boat, and it will hold only two at a time. The boat will not go across the river on its own. Because the men are of vastly different bulk, the time they take to cross the river varies. From slowest to fastest they take 10, 5, 2 and 1 minute respectively. When two are together, the boat crosses at the rate of the slower. What is the shortest time it takes to get all four across the river? (Answer tomorrow).

Yesterday's answer: (brackets indicate weights in same scale)

1. (R1+B1) : (R2+W1)

If they balance, try: (B2) : (W2). One will be heavier, if B2, then B2, R1 and W1 are the heavy balls.

If the first weighing does not balance, try: (B1) : (W2). The result will enable everything to be deduced.