contestada

The speed of the current is 7 mph. The boat travels three times slower against the current than with the current. What is the speed of the boat in still water?

Respuesta :

calculista
let
x--------> the speed of the boat in mph

we know that
going upstream the speed is x-7 
going downstream the speed is x+7 

let the distance be d 
time to go upstream = d/(x-7) 
time to go downstream = d/(x+7)

time going upstream is 3 times going downstream 
so
3d/(x+7) = d/(x-7) 
divide by d 
3/(x+7) = 1/(x-7) 
3x-21 = x+7 
2x = 28 
x = 14 

the answer is
the speed of the boat in still water is 14 mph

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