# 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

3 answers

###### Question:

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?

## Answers

b

Step-by-step explanation:

R = speed in still water

r + c = speed with current

r - c = speed against current

3.5(r + c) = 70

r + c = 20

c = 20 - r

4(r + c) = distance from pier to pier

5(r - c) = distance from pier to pier

4(r + c) = 5(r - c)

4(r + 20 - r) = 5(r - 20 + r)

80 = 10r - 100

180 = 10r

18 = r

The speed in still water is 18 km/hour

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

add them

2x = 36

x = 18 , then y = 2

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