SOLUTION: a motorboat maintained a constant speed of 10 miles per hour relative to the water in going 21 miles upstream and returning. the total time for the trip was 5.0 hours. use this inf

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Question 786914: a motorboat maintained a constant speed of 10 miles per hour relative to the water in going 21 miles upstream and returning. the total time for the trip was 5.0 hours. use this information to find the speed of the current
Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
boat speed = 10 mph
current speed = x mph

against current = 10 - x
with current = 10 + x

Total distance = 21 miles Total Time= 5.00 hours
Time upstream + time downstream = 5 hours

21 /( 10 - x )+ 21 /( 10 + x )= 5
21 ( 10 + x )+ 21 ( 10 - x )= 5.00 ( 100 - 1 X^2 )
210 + 21 x + 210 -21 x = 500 - 5 X^2
420 - -500 = -5 X^2
-80 = -5 X^2
-5 X^2= -80
X^2 = 16
X = +/- 4
speed of current = 4 mph