Let F = the downstream speed of the water.
Then the boat's upstream speed is: 15 - F
The boat's downstream speed is: 15 + F
Assume both the journeys mentioned take T hours, then using "speed x time = distance" we get:
Downstream journey: (15 + F)T = 140
Upstream journey: (15 - F)T = 35
Add the two formulae together:
(15 + F)T + (15 - F)T = 140 + 35
15T + FT + 15T - FT = 175
30T = 175
T = 35/6
Use one of the equations to find F:
(15 + F)T = 140
15 + F = 140/T
F = 140/T - 15
F = 140/(35/6) - 15
F = 24 - 15
F = 9
i.e. the downstream speed of the water is 9 kph
Therefore, the boat's speed downstream is 15 + F = 15 + 9 = 24 kph.
the answer is: *24kph*
