SOLUTION: Bob was driving home at an average speed of 40mph for 2 hours. How fast should he drive for the next 1 hour to have an average speed of 50mph

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Question 1155855: Bob was driving home at an average speed of 40mph for 2 hours. How fast should he drive for the next 1 hour to have an average speed of 50mph
Found 2 solutions by ikleyn, josgarithmetic:
Answer by ikleyn(52855) About Me  (Show Source):
You can put this solution on YOUR website!
.

If the average speed is 50 mph during the 3-hours trip, then the total distance is  (2+1)*50 = 150 miles.


In 2 hours, he just covered 2*40 = 80 miles.


Hence, he should cover the rest  150-80 = 70 miles in the last, third hour.


ANSWER.  He should drive at 70 mph.


Answer by josgarithmetic(39625) About Me  (Show Source):
You can put this solution on YOUR website!
                SPEED      TIME        DISTANCE

Was doing         40         2         40*2=80

Next hour         r          1           r

TOTALS                       3          r+80

Want average speed , 50 mph.
%28r%2B80%29%2F3=50
-
r%2B80=150
r=150-80
r=70