, 1998

Solution:

When Bobbo is midway across the river, he has travelled 50 feet. Going at a speed of 2 feet/s, this means that Bobbo has already been in the river for $\frac{50 \text{ feet}}{2 \text{ feet} / \mathrm{s}} = 25~\mathrm{s}$. Then he has traveled $5 \text{ feet} / \mathrm{s} \cdot 25~\mathrm{s} = 125$ feet down the river. Then he has $175$ feet $- 125$ feet $= 50$ feet left to travel downstream before he hits the waterfall.

Bobbo travels at a rate of 5 feet/s downstream. Thus there are $\frac{50~\mathrm{feet}}{5 \text{ feet} / \mathrm{s}} = 10~\mathrm{s}$ before he hits the waterfall. He still has to travel 50 feet horizontally across the river. Thus he must travel at a speed of $\frac{50 \text{ feet}}{10~\mathrm{s}} = 5$ feet $/ \mathrm{s}$. This is a 3 feet/s difference from Bobbo's original speed of 2 feet $/ \mathrm{s}$.