SOLUTION: A motorboat goes 8 miles downstream, on a river, in 20 minutes. It takes 30 minutes for this boat to go back upstream the same 8 miles. Find the speed of the river current (in mph)
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-> SOLUTION: A motorboat goes 8 miles downstream, on a river, in 20 minutes. It takes 30 minutes for this boat to go back upstream the same 8 miles. Find the speed of the river current (in mph)
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Question 870554: A motorboat goes 8 miles downstream, on a river, in 20 minutes. It takes 30 minutes for this boat to go back upstream the same 8 miles. Find the speed of the river current (in mph) assuming that the boat speed and river current speed remain constent. Answer by mananth(16946) (Show Source):
Distance against 8 miles distance with 8 miles
t=d/r against current -
8 / ( x - y )= 1/2
1/2 ( x - y ) = 1/3
1/2 x - 1/2 y = 8 ....................1
8 / ( x + y )= 1/3
1/3 ( x + y ) = 8
1/3 x + 1/3 y = 8 ...............2
Multiply (1) by 1/3
Multiply (2) by 1/2
we get
1/6 x + - 1/6 y = 2 5/8
1/6 x + 1/6 y = 4
1/3 x = 6 5/8
/ 1/3
x = 20 1/8 mph
plug value of x in (1) y
1/ 2 x - 1/ 2 y = 8
10 2/33 - 1/ 2 -10 2/33 = 8
- 1/ 2 y = 8
- 1/ 2 y = -2 2/33 mph
y = 4 4/33
boat speed 20 4/33 mph
current speed 4 4/33 mph
convert to decimal if required 4.12 mph
