Question 13066
Assume you know where they will meet and call it M. How long does it take Mary to get to M? The answer is 21 mi/hr times x hours. So, we have 
21x = M
How long does it take Sally to get to M? The answer is that she must travel the distance that takes her to M starting at 60 mi. And that is 60-M. So, we have
15x = 60-M
Substituting the value of M from the first equation into the second we get
15x = 60 - 21x
36x = 60
x = 5/3
This says that after 5/3 of an hour Mary will have travelled 35 miles (21 x 5/3) and
Sally will have travelled 25 miles (15 x 5/3).
Checking: 35 miles + 25 miles = 60 miles.