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