Andy cut a tuna sandwich and a chicken sandwich into a total of 21 same size-pieces. he cut the tuna sandwich into 15 more pieces than the chicken sandwich. andy ate 12 pieces of the tuna sandwich. what fraction of the tuna sandwich did he eat? can someone expand on how to answer it to get 1/3

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.)

I’m not sure what it is i sorry : ((