To find out what percent of his weekly allowance Fernando spent on snacks, you can use the formula:
\[ \text{Percentage} = \left( \frac{\text{Part}}{\text{Whole}} \right) \times 100 \]
In this case, the "Part" is the amount spent on snacks ($20), and the "Whole" is the total weekly allowance ($50).
\[ \text{Percentage} = \left( \frac{20}{50} \right) \times 100 \]
Calculating this gives:
\[ \text{Percentage} = 0.4 \times 100 = 40% \]
Therefore, Fernando spent 40% of his weekly allowance on snacks.