15. (10 points) A boat travels 16 kilometers downstream and 8 kilometers upstream in 4 hours; it also travels 12 kilometers downstream and 10 kilometers upstream in the same amount of time. Question: How many hours did it take for the boat to travel 24 kilometers downstream and then return?
【Solution】Solution: According to the problem, the time taken to travel downstream $16-12=4$ kilometers is equal to the time taken to travel upstream $10-8=2$ kilometers, which means in the same time, 4 kilometers were traveled downstream and 2 kilometers upstream, so the downstream speed is twice the upstream speed. Assuming the first voyage was entirely downstream:

Downstream speed: $(16+8 \times 2) \div 4=8$ (km/h)
Upstream speed: $8 \div(4-16 \div 8)=4$ (km/h),
The boat took $24 \div 8+24 \div 4=9$ hours to travel 24 kilometers downstream and then return.
The answer is 9.