Question 500351
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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