Question 177644
Since Russ is running at 7 mph, and Janet at 5 mph, in 1 hour, they would be 2 miles apart.
Using this info., we can then utilize proportions to figure out the time that it tales them to get to distances that are  {{{(1)/(2)}}}, or .5 miles apart. 
Therefore, we have: {{{(1)/(a)=(2)/(.5)}}}
2a  =  .5
a  =  .25


Therefore, it takes .25 hour, or 15 minutes for Russ and Janet to be  {{{(1)/(2)}}}, or .5 of a mile apart.


Checking:
In 15 mins, or .25 hr, Russ will be 1.75 (.25 x 7) miles away from the starting point.
In 15 mins, or .25 hr, Janet will be 1.25 (.25 x 5) miles away from the starting point.
Therefore, they will be .5 (1.75 – 1.25) of a mile apart, 15 minutes, or .25 hr after starting their run.