Answer:
9 shirts
Step-by-step explanation:
20% of the shirts were blue
20% of 30 is 6
6 shirts were blue
1/2 of the shirts were black
1/2 = 50%
50% of 30 is 15
15 shirts are black
15 + 6 = 21
30 - 21 = 9
(or you could add the percentages to get 70%, and get 30%. 30% of 30 is 9.)
Hope this helps!