Hello,
This problem can be simply solved by putting everything into fractions. Since there's a total of 50 shirts but 35 of them are medium sized then the fraction would be 35/50. You're trying to find the percentage so the 2nd fraction would be out of 100 so it would be ?/100.
35 ?
---- = ----
50 100
Now you diagonal multiplication then downwards division.
100 x 35=3500
3500/50=70
So your answer is 70%!! :D
Hope this helps a bunch!!!