Lets say the number of people over 25 is o (for old) and that the number of people 25 and younger are y (for young).

o = y/3; because there is one over 25 for every three 25 and under.

o + y = 8500; because there are 8500 enrollments in total

if we substitute o = y/3 into o + y = 8500, we get;

y/3 + y = 8500, this can be simplified to;

(4/3)y = 8500

y = 8500 / (4/3) = 8500 * 3 / 4

y = 6375

Now that we have a y value, we can easily work out the o value.

Substitute y = 6375 into o + y = 8500, we get;

o + 6375 = 8500

o = 8500 - 6375

o = 2125

lets check that these values are true;

o + y = 8500, so if we plug in the values for o and y...

2125 + 6375 = 8500, they do indeed add up to 8500

You can also check that o is one third of y;

o = y/3

2125 = 6375/3, o is indeed one third of y

So for the enrollments at a local university, out of 8500 students, 2125 were aged over 25 and 6375 were aged 25 or under.A