SOLUTION: Upstream, downstream. Junior�s boat will go 15 miles per hour in still water. If he can go 12 miles downstream in the same amount of time as it takes to go 9 miles upstream, then w

Question 285932: Upstream, downstream. Junior�s boat will go 15 miles per hour in still water. 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?
Answer by oberobic(2304) (Show Source): You can put this solution on YOUR website!
D = R*T, where D=distance, R=rate of speed, and T=time
.
S = speed in calm water = 15
C = current speed
S-C = speed going upstream
S+C = speed going downstream
.
T = D/R
.
Upstream...
T = 9/(S-C)
.
Downstream...
T = 12/S
.
Transitive property lets us eliminate T...which is good since we don't know what T is...
9/(S-C) = 12/S
.
Cross multiply...
9S = 12*(S-C) = 12S - 12C
.
Substitute S=15...
9(15) = 12(15) - 12C
.
135 = 180 - 15C
.
Add 15C to both sides...
15C + 135 = 180
.
Subtract 135 from both sides...
15C = 45
.
Divide both sides by 15
C = 3 = current speed
.
Checking the answer by checking the time up and downstream..
.
T = 9/(S-C) = 9/9 = 1 hr
.
T = 15/(S+C) = 15/(12+3) = 15/15 = 1 hr
.
Correct.
.
Answer:
The current is 3 mph.
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