Question 78224: Russ and Janet are running in the Apple Hill Fun Run. Russ runs at 7 mph, Janet at 5 mph. If they start at the same time, how long will it be before the are 1/2 mile apart?
The answer should be 15 min, but I'm not sure how to solve this...
Thank you for your help.
Answer by Edwin McCravy(20060) (Show Source):
You can put this solution on YOUR website!
Russ and Janet are running in the Apple Hill Fun Run.
Want to know what that is?
It happens every fall in Sacremento CA
http://www.applehillrun.org/race_info.html
Russ runs at 7 mph, Janet at 5 mph. If they start at the
same time, how long will it be before the are 1/2 mile apart?
You can do it in your head. They are separating at the rate
of 7-5 or 2 mph. So in an hour they would be 2 miles apart,
so in 15 minutes they will be half a mile apart.
Answer: 15 minutes.
But your teacher probably won't accept that way of doing it.
To make your teacher happy, make this DRT chart
DISTANCE RATE TIME
Russ
Janet
Let the time it takes them be t. So fill in
t for both their times.
DISTANCE RATE TIME
Rus t
Janet t
Now fill in their rates, 7 and 5
DISTANCE RATE TIME
Rus 7 t
Janet 5 t
Now use DISTANCE = RATE × TIME to
fill in the distances:
DISTANCE RATE TIME
Rus 7t 7 t
Janet 5t 5 t
Now we want to find the time when
their distances apart are 1/2 mile
Russ' distance MINUS Janet's distance = 1/2 mile
7t - 5t = 1/2
2t = 1/2
Multiply both sides by 2 to clear of fractions:
4t = 1
t = 1/4 hour
and 1/4 an hour equals 15 minutes.
Edwin
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