Let <em>f</em> and <em>s</em> denote the amounts of the <u>f</u>irst and <u>s</u>econd brands that the chef is going to use.
She wants to end up with 290 mL, so
<em>f</em> + <em>s</em> = 290
Each mL of the first brand contains 0.08 mL of vinegar, and each mL of the second contains 0.13 mL of vinegar. The final mixture should have a concentration of 12% vinegar, so that it contains 0.12 • 290 mL = 34.8 mL of vinegar, and
0.08<em>f</em> + 0.13<em>s</em> = 34.8
Solve for <em>f</em> and <em>s</em> :
<em>f</em> + <em>s</em> = 290 → <em>s</em> = 290 - <em>f</em>
0.08<em>f</em> + 0.13 (290 - <em>f </em>) = 34.8
0.08<em>f</em> + 37.7 - 0.13<em>f</em> = 34.8
0.05<em>f</em> = 2.9
<em>f</em> = 58
<em>s</em> = 290 - 58
<em>s</em> = 232