Jenna ordered 28 shirts for her soccer team seventy five percent of those shirts were size large how many large shirts did jenna order

Yup, it is 21 shirts that were large. Reason being is so: 

1) You have to set up a proportion in order to solve this equation. You know that there is a total of 28 shirts that Jenna ordered BUT 75% of them were large.

As you also know, percents are ALWAYS over a 100. Using this knowledge, let's set up our proportion: [use x to represent the amount of large shirts she ordered]

x      75
-- =
28   100 

2) To solve we will cross multiply! (First do 28 times 75, which gives you 2,100 and 100 times x, which gives you a 100 x).
As a result, you end up with:
100x= 2,100 

3) Now isolate x!!! 
100x=2,100
   
100      100   DIVIDE into 2,100

x= 21 

4) Your final answer is 21 large shirts!