It is 18/20 and in simplest form it is 9/10.
Steps:
1.You have to add all the shirts, 2+7+11. This is 20, this will become your denominator
2. See what portion is NOT BLACK, the non black at 7 and 11, once you add these it becomes 18. 18 is now your numerator.
3. You now have the fraction 18/20 which you can simplify into 9/10
Hope this helps :)