Answer:
2.44706 mph
Step-by-step explanation:
Given that :
Distance = rate * time
Rate of boat + rate of current = 4mph
If rate of current = c ;
Downstream:
85 = 4t
Upstream :
52 = (4 - c)t
85/4 = 52/(4 - c)
85(4 - c) = 52 * 4
340 - 85c = 208
-85c = - 132
c = 132 / 85
c = 1.55294
Rate of boat in still water :
4 - c
4 - 1.55294
= 2.44706 mph