A man covers 78 km upstream and 232 km downstream in 14 hours.  He also covers 65 km upstream and 87 km downstream in 8 hours.  Find the speed of the boat in still water.

21 km/h

\(\frac{78}{U}\) + \(\frac{232}{D}\) = 14 hours             [by Hit and trial as 78 and 65 both are divisible by 13]

\(\frac{65}{U}\) + \(\frac{87}{D}\) = 8 hours                 [So Let, U = 13]

\(\frac{65}{13}\) + \(\frac{87}{D}\) = 8

D = 29

or by solving equation

Speed of boat = \(\frac{U + D}{2}\) = \(\frac{13 + 29}{2}\) = \(\frac{42}{2}\) = 21 km/h