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Jenny drove to the gym with an average speed of 60 miles per hour. On

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Jenny drove to the gym with an average speed of 60 miles per hour. On  [#permalink]

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New post 01 Oct 2018, 07:20
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Jenny drove to the gym with an average speed of 60 miles per hour. On her way back she drove the same route with an average speed of 80 miles per hour. What was er average speed for the whole trip in mph?

a) 60

b) 68

c) 68 4/7

d) 70

e) 74 3/8

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Re: Jenny drove to the gym with an average speed of 60 miles per hour. On  [#permalink]

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New post 01 Oct 2018, 07:35
bettatantalo wrote:
Jenny drove to the gym with an average speed of 60 miles per hour. On her way back she drove the same route with an average speed of 80 miles per hour. What was er average speed for the whole trip in mph?

a) 60

b) 68

c) 68 4/7

d) 70

e) 74 3/8

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Direct formula...
\(\frac{2*a*b}{a+b}=\frac{2*60*80}{60+80}\)=960/14=480/7=68 4/7

C

Otherwise..
Let the distance be d..
d/60 +d/80 =3d+4d/240.. = total time taken
Average speed = 2d/(7d/240)=480/7

C
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Re: Jenny drove to the gym with an average speed of 60 miles per hour. On  [#permalink]

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New post 01 Oct 2018, 09:23
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bettatantalo wrote:
Jenny drove to the gym with an average speed of 60 miles per hour. On her way back she drove the same route with an average speed of 80 miles per hour. What was er average speed for the whole trip in mph?

a) 60

b) 68

c) 68 4/7

d) 70

e) 74 3/8


Another approach is to assign a "nice" value (one that works well with 60 mph and 80 mph) to the distance to the gym.
So, let's say the distance is 480 miles (a long commute indeed!)

Average speed = (total distance traveled)/(total travel time)

TOTAL distance = 480 miles + 480 miles = 960 miles

Now let's calculate the TOTAL travel time.

time = distance/speed
So, travel time TO the gym = 480/60 = 8 hours
And travel time FROM the gym = 480/80 = 6 hours
So, TOTAL travel time = 8 hours + 6 hours = 14 hours


So, average speed = 960 miles/14 hours
= 480/7 miles per hour
= 68 4/7 miles per hour

Answer: C

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Re: Jenny drove to the gym with an average speed of 60 miles per hour. On  [#permalink]

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New post 02 Oct 2018, 00:41
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bettatantalo wrote:
Jenny drove to the gym with an average speed of 60 miles per hour. On her way back she drove the same route with an average speed of 80 miles per hour. What was er average speed for the whole trip in mph?

a) 60

b) 68

c) 68 4/7

d) 70

e) 74 3/8

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If the distance from home to gym is D, then total distance =2D
time from Home to Gym T1=D/60
time from Gym to home T2=D/80

average speed = total distance/(T1+T2)
= 2D/(D/60+D80)
=68 4/7
answer is C
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Re: Jenny drove to the gym with an average speed of 60 miles per hour. On  [#permalink]

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New post 03 Oct 2018, 16:42
bettatantalo wrote:
Jenny drove to the gym with an average speed of 60 miles per hour. On her way back she drove the same route with an average speed of 80 miles per hour. What was er average speed for the whole trip in mph?

a) 60

b) 68

c) 68 4/7

d) 70

e) 74 3/8


We can let the distance each way = 240 miles and use the formula: average rate = total distance/total time:

average rate = 480/(4 + 3) = 480/7 = 68 4/7

Answer: C
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Re: Jenny drove to the gym with an average speed of 60 miles per hour. On &nbs [#permalink] 03 Oct 2018, 16:42
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