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
add them
2x = 36
x = 18 , then y = 2