2 cakes - Theresa's and Joe's
Theresa's cake had 6 pieces after she cut it. (2 times the size of Joe's pieces)
Joe's cake had 12 pieces after he cut it. (1/2 the size of Theresa's pieces)
We know that 8/12ths of ONE cake were eaten and that Joe ate 2 of his pieces.
We want to know how many pieces Theresa ate of her cake. Keeping in mind that her pieces are equal to 2 of Joe's pieces we can solve this question.
8/12 eaten total
if 2/12 by Joe
then 8-2 = 6, 6/12 by Theresa
(BUT: Theresa's pieces were twice the size of Joe's so we will divide by 2)
6/12 = 3/6
Answer: Theresa ate 3 pieces of her cake