Show a complete solution to problem Juniors boat will go 15

Show a complete solution to problem.
Junior\'s boat will go 15 miles per hour in stillwater. If he can go 12 miles downstream in the same amount of time as it takes to go 9 miles upstream, then what is the speed of the current?

Solution

d=st d=istance, s=speed, t=time
t=d/s
Let c=speed of current
12/(15+c)=9/(15-c) time going downstream = time going upstream
12(15-c)=9(15+c) multiply each side by the denominators to eliminate fractions.
180-12c=135+9c
-21c=-45
c=15/7=2.14... mph speed of current