Question

John has a boat that will travel at the rate of 15 kph in still water. he can go upstream for 35 km in the same time it takes to go 140 km downstream. how fast is his boat traveling when he goes downstream? 22 kph 24 kph 26 kph

Answer 1

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*