A man can go on a motor boat with a speed of 15 km/hour

Solution

Let, the speed of boat be b km/hour and speed of current be s km/hour  Average speed = 2xy/(x+y)  (here, x is speed of downstream and y is speed of upstream)                            = (2 (b+s)(b-s))/((b+s+b-s))                            = (b² - s²)/b                             = (225 - 25)/15 = 200/15 = (40/3) km/hour Alternate Method :- Average speed = (Total distance covered)/(total time taken)                             = (2×60)/9  = 120 /9 = 40/3 kmph