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Linda drove 50 miles from San Jose to San Francisco in 90 minutes. For the first 30minutes, without much traffic, her average speed was 80 miles an hour. In the second 30 minutes, due to a traffic accident along US 101, her average speed dropped to 25miles an hour. She was stuck in traffic for the last 30 minutes near San Francisco.What was her average speed, in miles per hour, during the last 30 minutes?

 Jun 6, 2022
 #1
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If she was going 80 miles per hour for half an hour she would have gone 40 miles leaving 10 miles left. if for half an hour she went 25 miles per hour she would have gone 12.5 miles which is more than 10...... So I dont know if im doing something wrong, but this doesnt seem answerable?

 Jun 6, 2022
 #2
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Change the distance between San Francisco and San Jose from 50 miles to 60 miles! [50 does NOT work].

 

Let her speed in the last 30 minutes==S

 

[80 x 1/2] + [25 x 1/2] + [1/2S] ==60, solve for S

 

40  +  12.5  +  1/2S == 60

 

1/2S ==60 - 40 - 12.5==7.5

 

S ==7.5  x  2 ==15 mph - her average the last 30 minutes.

 Jun 6, 2022

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