The answer to this is that John's previous salary was right about $39,130.  Here is how you can find this answer:

Set up the equation like this:

45000/x = 115/100

This is because the relationship between 45000 and x (the original salary) is the same as the relationship between 115 and 100 (the new salary is 115% of the old).

Now you can cross multiply.  We will multiply 45000 by 100 and 115 by x.  That leaves us with

4,500,000 = 115x

divide both sides by 115 and you get

x = 39,130.43

Let us assume that John's salary before the raise was x

Then they gave him a 15% raise on his salary of x

Then, his current salary= original salary + The raise

45,000 = x + 0.15 x

45,000 = 1.15 x

==> x = 45,000/1.15 = 39,130  (approx.)