The task is to generate the correct answer for a given math problem.

Problem: salad dressing p is made up of 30 % vinegar and 70 % oil , and salad dressing q contains 10 % vinegar and 90 % oil . if the two dressings are combined to produce a salad dressing that is 18 % vinegar , dressing p comprises what percentage of the new dressing ?
40 %