I am having trouble understanding a math question

Question

A chef plans to mix 100%

 vinegar with Italian dressing. The Italian dressing contains 13%

 vinegar. The chef wants to make 150

 milliliters of a mixture that contains 42%

 vinegar. How much vinegar and how much Italian dressing should she use?

Answer 1

X is the amount of salad dressing...

The amount of vinegar is then:

0.13X + (150-x) = 0.42*150

0.13X + 150- x = 63

-0.87x + 150 = 63

-0.87X = -87

X=100

100 ml of the Italian dressing

50 mL of vinegar