Given:
1.Total pieces:21.
2.Tuna pieces = 15 more than Chicken pieces.
Supposing,
Tuna - y, chicken - x.
x + y = 21 (from 1)
y = x +15 (from 2)
x + (x+15) = 21 (from 1 and 2)
2x + 15 = 21; 2x= 21-15; *x= 3.*
**So, pieces of chicken(x) = 3, tuna = 21-3=18.**
Andy ate 12 pieces of Tuna, out of 18.
So, he ate 12/18 = 2/3 fraction of it.
(1/3 is the amount of fraction left, not what he ate.)