Speed of stream 'R' is 4 km/h more than speed of stream 'S'. Sum of upstream speed of a boat in 'R' and its downstream speed in 'S' is 42 km/h. Time taken by boat to cover 216 km while travelling downstream in 'R' is 6 hours. Find the difference between the upstream distance covered by boat in 'R' in 3 hours and downstream distance covered by boat in 'S' in 2 hours.

Let the speed of stream 'S' be 'y' km/h and speed of boat in still water be 'v' km/h. Speed of stream 'R' = (y + 4) km/h Upstream speed of boat in 'R' = (v - y - 4) km/h Downstream speed of boat in 'S' = (v + y) km/h So, v - y - 4 + v + y = 42 Or, 2v - 4 = 42 Or, 2v = 46 Or, v = 23 Downstream speed of boat in 'R' = 23 + y + 4 = (27 + y) km/h So, 27 + y = 216/6 = 36 Or, y = 9 Upstream speed of boat in 'R' = 23 - 9 - 4 = 10 km/h Downstream speed of boat in 'S' = 23 + 9 = 32 km/h Required difference = |32 × 2 – 10 x 3|
= |64 – 30|
= 34 km