A motorboat, travelling at the same speed, can cover 25 km upstream and 39 km downstream in 8 hours. At the same speed, it can travel 35 km upstream and 52 km downstream in 11 hours. The speed of the steam is

Let the upstream speed be x km/hr and the downstream speed be y km/hr. The given conditions yield the equations 25/x + 39/y = 8 and 35/x + 52/y = 11. Multiplying the first equation by 4 gives 100/x + 156/y = 32, and multiplying the second by 3 gives 105/x + 156/y = 33. Subtracting these produces 5/x = 1, so x = 5. Substituting x into the first equation yields 5 + 39/y = 8, meaning 39/y = 3 and y = 13. The speed of the stream is half the difference of the downstream and upstream speeds, calculated as (13 - 5) / 2 = 4 km/hr.

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