To find the percentage of medium-sized jerseys, you can use the formula:
\[ \text{Percentage} = \left( \frac{\text{Number of medium-sized jerseys}}{\text{Total number of jerseys}} \right) \times 100 \]
In this case, the number of medium-sized jerseys is 15 and the total number of jerseys is 25.
Plugging in the numbers:
\[ \text{Percentage} = \left( \frac{15}{25} \right) \times 100 \]
Calculating the fraction:
\[ \frac{15}{25} = 0.6 \]
Now, multiplying by 100 to get the percentage:
\[ 0.6 \times 100 = 60 \]
Thus, 60% of the jerseys are medium-sized jerseys.