Question 1059753
Let x = speed of boat in still water
Let y = speed of the current
then
(x+y) = speed with the current
and
(x-y) = speed against the current
:
Write a distance equation for each trip: 
Distance = time * speed
3(x + y) = 87
4(x - y) = 76
:
x + y = 29
x - y = 19
Add these two equations and you get:
x+x = 2x
y+-y = 0
29+19 = 48
Write your new equation:
2x = 48
x = 24 mph is the speed in still water
:
The speed of the current:
We said x+y = 29, 24+y = 29, y = 5 this is the speed of the water
Or you can do it with x-y=19; y = x-19, y = 24-19 = 5 again, 5 is the speed of the water whether going up river or down river (of course, downriver it pushes you faster and upriver it slows you down)