A boat takes a total of 1.5 hours to cover 20km upstream

Solution

Speed of the boat in still water = 120 ÷ 4.8 = 25km/h Let the speed of the stream be 'x' km/h. ATQ. {20/ (25 - x)} + {15/(25 + x)} = (3/2) Or {20 X (25 + x) + 15 X (25 - x)} = (3/2) X (25 +x) X (25 - x) Or {(500 + 20x + 375 - 15x) X 2} = 3 X (625 -x²) Or, 1750 + 10x = 1875 - 3x² Or 3x² + 10x - 125 = 0 Or 3x² + 25x -15x - 125 = 0 Or x (3x + 25) - 5(3x + 25) = 0 Or (x - 5) (3x + 25) = 0 So, x = 5 or x = -(25/3) Since the speed of the stream cannot be negative therefore, speed of stream =5km/h so required time =D/s= 32/5 =6.4hours