Answer: Approximately 54.7826 mph
As a fraction, this average speed is exactly 1260/23 mph.
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Work Shown:
"Alice drove 30 miles...at 45 mph". This means
t = d/r
t = 30/45
t = 2/3
She drove for 2/3 of an hour, which is (2/3)*60 = 40 minutes.
Then "she drove home at 70 mph". She drove the same distance, meaning,
t = d/r
t = 30/70
t = 3/7
She drove for 3/7 of an hour, which is roughly (3/7)*60 = 25.714 minutes.
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Add up the time values
(2/3 hr) + (3/7 hr)
(2/3 + 3/7) hr
(14/21 + 9/21) hr
23/21 hr
She spent 23/21 of an hour driving going to the auto shop and then back home. The total distance she traveled was 30+30 = 60 miles.
Use these values to find the speed
r = d/t
r = d divided by t
r = 60 divided by (23/21)
r = (60/1) divided by (23/21)
r = (60/1) * (21/23)
r = (60*21)/(1*23)
r = 1260/23
r = 54.7826
Her average speed for the round trip is about 54.7826 mph.
