math
posted by Adam .
River Current A boat can travel 36 miles upstream in the same time that it can travel 54 miles downstream. If the speed of the current is 3 miles per hour, find the speed of the boat without a current.

let speed of boat in still water be x mph
upstream:
speed = x5
time = 36/(x5)
downstream:
speed = x+5
time = 54/(x+5)
the two times are equal, so
36/(x5) = 54(x+5)
54x  270 = 36x + 180
18x = 450
x = 25
The speed of the boat without a current is 25 mph 
A canoe can travel 18 mph in still water and 22 miles downstream in the same amount of time that it takes to travel 14 mph upstream. Find the speed of the current in the water.

a person can copy the final exam in 28 minutes using a new copy machine. using an old copy machine, it takes 70 minutes. if both machines are used, how long will it take to complete the job