A man can row 90 km downstream and return in a total of 8 hours. If the speed of the boat in still water is four times the speed of the current, then the speed of the current is?

6 km/hr

Ratio of speed of boat and current = 4  :  1

ATQ,     \(\frac{90}{4R + 1R}\) + \(\frac{90}{4R - 1R}\) = 8

            \(\frac{18}{R}\) + \(\frac{30}{R}\)  = 8

                                         \(\frac{48}{8}\) = R

                                                                R = 6

Speed of current = 1R = 1 × 6 = 6 km/hr