Hi,

33 1/3% is the fraction 1/3 so the t-shirt sale was one third off. There was one third off and hence Joe paid two thirds of the original price. Hence $12 is two thirds of the original price and thus $6 is one third of the original price. If $6 is one third of the original price then the original price must have been $18.

Check:

Original price $18.
33 1/3% of $18 is 3 of $18 which is $6.
Joe paid $18 - $6 = $12

There is a more algebraic wy to solve this problem. Let $P be the original price then since Joe paid two thirds of the original price

2/3 × $P = $12

Multiply both sides by 3 to get

2 × $P = $36

divide both sides by 2 and you get

$P = $18

I hope this helps,
Penny