Step 1: Calculate the total cost per cup.
The total cost of supplies is $ 20, and the boy can make 500 cups, so the cost per cup is:
\[
\frac{20}{500} = 0.04 \text{ dollars per cup}.
\]
Step 2: Calculate the total cost including tax.
Each cup has a 4% tax, so the total cost per cup is:
\[
0.04 + 0.04 \times 0.04 = 0.0416 \text{ dollars per cup}.
\]
Step 3: Calculate the number of cups needed to break even.
To break even, the revenue from selling \( x \) cups must equal the total cost of supplies:
\[
x \times 0.25 = 20.
\]
Solving for \( x \):
\[
x = \frac{20}{0.25} = 80 \text{ cups}.
\]
Thus, the boy must sell 80 cups to break even.