math

A motorboat covers the distance from one pier to the other pier in 4 hours and the way back in 5 hours. What is the speed of the boat in still water, if it covers 70 km with the current in 3.5 hours?

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1. speed of boat in still water ---- x km/h
speed of current ------ y km/h

distance with the current = 4(x+y)
distance against the current = 5(x-y)

we know that 3.5(x+y) = 70
x+y = 20

but 4(x+y) = 4(20) = 80
then 5(x-y) = 80
x-y = 16

x+y=20
x-y=16
2x = 36
x = 18 , then y = 2

state the conclusion

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2. I didn't ask

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