SOLUTION: Can someone clarify why in this question 5x is subtracted from 7x and not added? Does it have anything to do with the miles apart is 1/2 mile-under 1 mph. Also, is there a way

Algebra ->  Customizable Word Problem Solvers  -> Travel -> SOLUTION: Can someone clarify why in this question 5x is subtracted from 7x and not added? Does it have anything to do with the miles apart is 1/2 mile-under 1 mph. Also, is there a way       Log On

Ad: Over 600 Algebra Word Problems at edhelper.com


   



Question 1120731: Can someone clarify why in this question 5x is subtracted from 7x and not added?
Does it have anything to do with the miles apart is 1/2 mile-under 1 mph.
Also, is there a way to know when to add or subtract?
Question
Sally and Lucy are running in the marathon. Sally runs at 7 mph and Lucy runs at 5 mph. If they start at the same time, how long will it be before they are 1/2 mile apart?
7x-5x=1/2
x=1/4 hour; 15 minutes
Thanks in advance.

Answer by solver91311(24713) About Me  (Show Source):
You can put this solution on YOUR website!


It is because they are going the same direction. Two objects going the same direction separate at a rate equal to the DIFFERENCE of their speeds.


John

My calculator said it, I believe it, that settles it