To solve 
We use total derivative
Divide both sides with 
Multiply and divide 
and 
on relevant components on the RHS:
is the cross-price elasticity (unweighted, constant-output) of and 
, denote it as 
is also the elasticity of and Q. From 1) to 3) in the note of restriction, we know is linear homogeneous in Q, or the percentage change of equals the percentage change of Q, so 
Multiply 100 on both side and rewrite in percentage change form (lower case), we have:
