The problem says that the ratio of fluid ounces of water to fluid ounces of tomato paste is 3:4, which means for every 3 ounces of water you add 4 more ounces of tomato paste, then the sauce is compounded by a total of 3+4=7 fluid ounces of both ingredients.
Then, the proportion is:
[tex]\frac{4\text{ fluid ounces of tomato paste}}{\text{7 fluid ounces of sauce}}[/tex]
Then, to find how many fluid ounces of tomato paste do you need to make 35 fluid ounces of sauce you can solve the next equation:
[tex]\begin{gathered} \frac{4}{7}=\frac{x}{35} \\ \text{ Multiply both sides by 35} \\ \frac{4\cdot35}{7}=\frac{x\cdot35}{35} \\ \text{Simplify} \\ 4\cdot5=x \\ x=20 \end{gathered}[/tex]
Then, the answer is you need 20 fluid ounces of tomato paste to make 35 fluid ounces of sauce.
