The 1/3 full one which is twice as large equals 2/3 of a smaller one
so you have 2/3 + 1/2 = 7/6 of smaller one of 3 yr old wine
you basically are filling THREE casks (the smaller one PLUS the 2x bigger one)
the part that will be 3 yr old will be 7/6 out of 18/6 (three casks)
(7/6) / (18/6) = 7/18ths of the new mixture will be 3 yr old wine.