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

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