Let the volume expansion of liquid be δ and linear expansion of tin be αt.
Apparent expansion = real expansion of vessel + real expansion of liquid
Therefore, X=3α+δ ...(1) and
Y=3αt+δ ...(2)
On rearranging equations (1) and (2), we get,
δ=X−3α...(1)and
δ=Y−3α...(2)
Equating (1) and (2)
X−3α=Y−3αt
∴3αt=Y−X+3α
∴αt=Y−X+3α3