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Answer:
7.5mph
Step-by-step explanation:
Let the average speed of the boat to be x mph.
Total Distance = Avg. Speed x Total Time
60 = ((x+6) + (x-6))(4)
60 = (x+6+x-6)(4)
(2x)(4)=60
2x=60/4
2x = 15
x = 15 /2
x = 7.5mph
Answer:
15 +√261 ≈ 31.155 mph
Step-by-step explanation:
The time for the trip can be found by dividing the distance by the speed.
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The relation between time, speed, and distance is ...
time = distance/speed
The speed for the downstream leg was the sum of the boat speed (b) and 6 mph. The speed for the upstream leg was the difference. So the total trip time was ...
4 = 60/(b +6) +60/(b -6) . . . total time is the sum of the times for the legs
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Multiplying by the product of the denominators, we have ...
4(b +6)(b -6) = 60(b -6) +60(b +6)
b² -36 = 30b . . . . divide by 4 and simplify
b² -30b = 36 . . . . put in a form useful for completing the square
(b -15)² = 36 +15² . . . . . complete the square
b -15 = √261 . . . . . . . . take the square root (negative root is extraneous)
b = 15 +√261 ≈ 31.155 . . . . add 15
The average speed of the boat was about 31.155 mph.