Let x = the rate in still water and y = the rate of the current.
Distance = rate x time
Set up two equations:
Upstream: 7(x-y) = 385
Simplify by dividing both sides by 7:
x-y = 55
Downstream 4(x+y) = 420
Simplify by dividing both sides by 4:
x + y = 105
Now add both equations together:
x - y = 55
x +y = 105
= 2x = 160
Divide both sides by 2:
x = 80
The rate is 80 km per hour in still water.
Now solve for Y, which is the current:
x- y = 55
Replace x with 80:
80-y = 55
y = 25 km/hr ( cuurent rate)
