Answer: 33 1/3%
Explanation:
Percentage increase of the shirt will be calculated as:
= Price Increase / Old price × 100%
Price Increase = $20 - $15 = $5
Old price = $15
Percent increase = Price Increase / Old price × 100%
= 5/15 × 100
= 1/3 × 100
= 33 1/3%