Let us assume that,
Milk consumed by family initially =x
Price of milk initially =y
Total expenditure on milk =xy
There's a 15% increase in price of milk
New price of milk =1.15y
Also, a 15% reduction in consumption of milk by family
⇒ Milk consumed by family =0.85x
∴ Total expenditure on milk =1.15y×0.85x
=0.9775xy
% Decrease in expenditure =xy−0.9775xyxy×100=2.25%