It took a crew 80 min to row 3 km upstream and back again. If the rate of flow of the stream was 3km/h, what was the rowing rate of the crew?
Total distance is 3 km (1.5 km each way)
Total time is 4/3 hr (80 min)
Rate of steam is 3 km/hr
rate of boat downstream = b + 3
rate of boat upstream = b - 3
$\displaystyle d = rt$
$\displaystyle t=\frac{d}{r}$
$\displaystyle \frac{1.5}{b+3} + \frac{1.5}{b-3}=\frac{4}{3}$
Does that sound reasonable?