SOLUTION: I need to know the formula to get this answer. Mike drove uphill to his school and it took 3 hours. He returned downhill only going 5mph faster and made it in two hours. What i

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Question 500351: I need to know the formula to get this answer.
Mike drove uphill to his school and it took 3 hours. He returned downhill only going 5mph faster and made it in two hours. What is the average rate of speed?
Found 2 solutions by Alan3354, ankor@dixie-net.com:
Answer by Alan3354(69443) (Show Source):
You can put this solution on YOUR website!
I need to know the formula to get this answer.
d = r*t is the formula
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Mike drove uphill to his school and it took 3 hours. He returned downhill only going 5mph faster and made it in two hours. What is the average rate of speed?
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d = r*t
d = r*3
d = (r+5)*2
3r = 2(r+5)
r1 = 10 mi/hr uphill
r2 = 15 mi/hr downhill
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Here's a formula:
avg speed = 2*r1*r2/(r1 + r2)
= 2*10*15/25 = 300/25
= 12 mi/hr
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Notice that the distance is irrelevant, but they have to be equal for the formula to apply.
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A different method, without the formula:
d = 10*3 = 30 miles
round trip = 60 miles
Total time = 3 hr + 2 hr = 5 hours
Avg speed = 60/5 = 12 mi/hr same answer
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Answer by ankor@dixie-net.com(22740) (Show Source):
You can put this solution on YOUR website!
Mike drove uphill to his school and it took 3 hours.
He returned downhill only going 5 mph faster and made it in two hours.
What is the average rate of speed?
:
Find the one-way distance first
let d = the one-way distance
:
Write a speed equation; speed = dist/time
:
up speed + 5 mph = down speed
+ 5 =
multiply by 6, to clear the denominators
2d + 6(5) = 3d
30 = 3d - 2d
d = 30 mi one way
:
Find the average speed for the round trip (60 mi)
Total time for the round trip: 2 + 3 = 5 hrs
:
write a speed equation, speed = dist/time
:
= 12 mph is the average speed for the round trip