SOLUTION: Lisa drove 60 miles on a highway, then increased her speed by 20 miles per hour for 240 miles. The second part of the drive took 2 hours longer than the first part. Choose the most

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Question 1086474: Lisa drove 60 miles on a highway, then increased her speed by 20 miles per hour for 240 miles. The second part of the drive took 2 hours longer than the first part. Choose the most reasonable answer for her average speed.
Answer by josgarithmetic(39625) About Me  (Show Source):
You can put this solution on YOUR website!
               SPEED      TIME           DISTANCE
part one          r       60/r               60
part two        r+20      240/(r+20)         240
difference                  2


240%2F%28r%2B20%29-60%2Fr=2
-
120%2F%28r%2B20%29-30%2Fr=1
120r-30%28r%2B20%29=r%5E2%2B20r
120r-30r-600=r%5E2%2B20r
90r-600=r%5E2%2B20r
r%5E2-70r%2B600=0
discrim, 4900-4%2A600=4900-2400=2500

r=%2870%2B-+50%29%2F2
r=10 or r=60

Taking r=60,
part one time, 1 hour
part two time, 3 hours
TOTAL TIME, 4 hours
-
Average speed, 300%2F4=highlight%2875%29