40% of the jerseys are medium sized,
That is because to find the percentage do,
20/50 = 0.40
This shows that 40% of the jerseys are medium
You can also do this:
20 to 50 (Ratio of medium to total jersey)
Multiply by 2 to both sides to get 100 for the total jerseys,
40 to 100
This also shows that 40% of the jerseys are medium sized