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?PLZ SHOW WORK AND DO NOT ESTIMATE

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