SOLUTION: A boat can go 45 mph in still water. It takes as long to go 630 miles upstream as it does to go downstream 720 miles. How fast is the current?

Question 264590: A boat can go 45 mph in still water. It takes as long to go 630 miles upstream as it does to go downstream 720 miles. How fast is the current?
Found 2 solutions by checkley77, Ruli:
Answer by checkley77(12844) (Show Source): You can put this solution on YOUR website!
D=RT
630=(45-C)T
T=630/(45-C)
720=(45+C)T
T=720/(45+C) BECAUSE THE 2 TIMES ARE THE SAME SET THE 2 EQUATIONS EQUAL & SOLVE FOR C (THE CURRENT).
630/(45-C)=720/(45+C) CROSS MULTIPLY.
720(45-C)=630(45+C)
32,400-720C=28,350+630C
-720C-630C=28,350-32,400
-1,350C=-4,050
C=-4,050/-1,350
C=3 MPH. IS THE SPEED OF THE CURRENT.
PROOF:
630/(45-3)=720/(45+3)
630/42=720/48
15=15

Answer by Ruli(21) (Show Source): You can put this solution on YOUR website!
630 miles upstrem = 630/(45-x)
720 miles downstream 720/(45+x)
630/(45-x) = 720/(45+x)
Multiply each side by (45-x)(45+x)
630(45+x) = 720(45-x)
28,350 + 630x = 32,400 - 720x
28,350 + 630x + 720x = 32,400 - 720x + 720x
28,350 + 1350x - 28,350 = 32,400 - 28,350
1350x = 4050
Divide each side by 1350
x = 3
The speed of the currecnt is 3 mph
Check:
630/(45-3) = 720/(45+3)
630/(42) = 720/(48)
15 = 15
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