A boat goes a distance of 4 km upstream in 2 hours and the same distance downstream in 20 minutes. How long will it take to go $10\frac{1}{2}$km in still water?

$1\frac{1}{2}$

Given,

Upstream speed = 12 km/hr

Downstream speed = 2 km/hr,

Speed of boat = (12 + 2) $\div$ 2 = 7 km/hr

Require time to cover 21/2 km in still water = (\(\frac{21}{2}\))$\div$ 7 = (\(\frac{21}{14}\)) = $1\frac{1}{2}$