Answer
To make 20 brownies, we need $\frac{5}{6}$ cups of butter, 6$\frac{1}{4}$ or 6.25 ounces of unsweetened chocolate, 1$\frac{7}{8}$ cups of sugar, 2$\frac{1}{2}$ teaspoons of vanilla, 2$\frac{1}{2}$ eggs, and 1$\frac{1}{4}$ cups of flour.
Work Step by Step
The multiplier we need to convert the ingredient amounts from 16 brownies in the original recipe to 20 brownies is: $\frac{20}{16}$ or, in reduced form, $\frac{5}{4}$. Note: all mixed numerals must be converted to improper fractions prior to being multiplied.
Butter: $\frac{2}{3}$ x $\frac{5}{4}$ which is $\frac{10}{12}$= $\frac{5}{6} cups$.
Unsweetened chocolate: $\frac{5}{1}$ x $\frac{5}{4}$ which is $\frac{25}{4}$. This is 6$\frac{1}{4}$ or 6.25 ounces.
NOTE: Before converting the amounts for sugar, we must first change 1$\frac{1}{2}$ cups to $\frac{3}{2}$ cups.
Sugar: $\frac{3}{2}$ x $\frac{5}{4}$ which means we need $\frac{15}{8}$ = 1$\frac{7}{8}$ cups
NOTE: Whole numbers are written over 1 so we can multiply them.
Vanilla: $\frac{2}{1}$ x $\frac{5}{4}$ = $\frac{10}{4}$ = 2$\frac{1}{2}$ teaspoons.
Eggs: $\frac{2}{1}$ x $\frac{5}{4}$ = $\frac{10}{4}$ = 2$\frac{1}{2}$ eggs.
Flour: $\frac{1}{1}$ x $\frac{5}{4}$ = $\frac{5}{4}$ = 1$\frac{1}{4}$ cups.
