Amotorboat 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? (its not 20 i tried)

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

1. 8 or -82. 14 or -143. -11, -3, 0, 7, 10