SOLUTION: I travel 45 miles by car. It runs out of gas. I walk 3 miles. It takes me a total of 90 minutes. I travel 15 times faster in a car than walking. What is my driving speed.

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Question 454779: I travel 45 miles by car. It runs out of gas. I walk 3 miles. It takes me a total of 90 minutes. I travel 15 times faster in a car than walking. What is my driving speed.
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
There are 2 different rates, so you need 2 different equations:
Let +r+ = driving speed
Let +t+ = time spent driving
In car:
+d+=+r%2At+
(1) +45+=+r%2At+
Walking:
(2) +3+=+%28r%2F15%29%2A%281.5+-+t%29+ ( units in miles, hrs, and miles/hr )
There are 2 equations and 2 unknowns, so it's solvable
(2) +3+=+%28r%2F15%29%2A%281.5+-+t%29+
(2) +3+=+r%2F10+-+%281%2F15%29%2Ar%2At+
Substitute (1) into (2)
(2) +3+=+r%2F10+-+45%2F15+
(2) +3+=+r%2F10+-+3+
(2) +r%2F10+=+6+
(2) +r+=+60+ mi/hr
check answer:
(1) +45+=+r%2At+
(1) +t+=+45%2F60+
(1) +t+=+.75+ hrs
and
(2) +3+=+%2860%2F15%29%2A%281.5+-+.75%29+
(2) +3+=+4%2A.75+
(2) +3+=+3+
OK