Answer: vinegar: 30 mL
Italian dressing: 290 mL
Explanation:
100% vinegar + 4% Italian dressing
He wants 320 mL of mix with 13% vinegar → 320*0.13 = 41.6 mL
So, the mixture must have 41.6 mL of vinegar and 278.4 mL of other ingredients of the Italian Dressing (-vinegar). This way:
Let's call Italian dressing x and vinegar y:
4%x + 100%y = 41.6
96%x = 278.4
0.96x = 278.4
x = 278.4/0.96
x = 290
So, 290 mL of Italian dressing.
As he wants 320 of mixture, 320 - 290 = 30 mL of vinegar.
Italian dressing: 290 mL (11.6 mL vinegar)
100% Vinegar: 30 mL
Vinegar = 30 + 11.6 = 41.6 mL
320.13% = 41.6 mL (ok)