A boat travels 42 km against the current and 72 km with

Solution

Speed of the boat in still water = 56/4 = 14 km/h Let the speed of the stream = x km/h So according to question: 42/(14 – x) + 72/(14 + x) = 8 294 + 21x + 504 –36x = 784 – 4x² 4x² – 15x + 14 = 0 4x² – 8x – 7x + 14 = 0 4x(x – 2) – 7(x – 2) = 0 (4x – 7)(x – 2) = 0 x = 2, 7/4 = 2, 1.75 As the speed of the stream is greater than 1.75, the value of x = 2 So the downstream speed and upstream speed of the boat are 16 km/h and 12 km/h respectively. So the desired percentage = (16 – 12)/16 × 100 = 25%