Hi Shanah.

If you want 360mL of solution which is 75% acetic acid, then how many mL of it is pure acetic acid? Answer: 75% x 360mL. Let's call this quantity A.

So that's how much acetic acid you need. Your stock is 99% acetic acid, so to get A mL of acetic acid, you need 99% x S = A where S is the volume of the stock acid. We can rewrite this as S = A / 99%.

Substituting in the expression for A into this, we get:

S = (75% x 360 mL) / 99%

which gives the number of mL of stock acid to pour. Then just top it up to the 360mL level with distilled water: W = 360mL - S.

Hope this helps,
Stephen La Rocque.