Let x be the amount of tomato paste so that \(\displaystyle{35}−{x}\) is the amount of water, both in fluid ounces. Using the ratio water/tomato paste, we write the proportion:

\(\displaystyle{\frac{{{35}-{x}}}{{{x}}}}{)}={\frac{{{3}}}{{{4}}}}\)

Solve for x:

\(\displaystyle{4}{\left({35}−{x}\right)}={3}{x}\)

\(\displaystyle{140}−{4}{x}={3}{x}\)

\(\displaystyle{140}={7}{x}\)

\(\displaystyle{x}={20}\)

So, you need 20 fluid ounces of tomato paste.

\(\displaystyle{\frac{{{35}-{x}}}{{{x}}}}{)}={\frac{{{3}}}{{{4}}}}\)

Solve for x:

\(\displaystyle{4}{\left({35}−{x}\right)}={3}{x}\)

\(\displaystyle{140}−{4}{x}={3}{x}\)

\(\displaystyle{140}={7}{x}\)

\(\displaystyle{x}={20}\)

So, you need 20 fluid ounces of tomato paste.