The price of milk has increased by 20%. To keep the expenditure unchanged, current consumption must be reduced by:

Price increased by 20%, so new price becomes 120% of original.

Let original price = P and original consumption = Q.

So original expenditure = P × Q.

After increase, new price = $1.2P$

To keep expenditure same:
$1.2P \times \text{new consumption} = P \times Q$

So,
new consumption = $1/1.2 = 5/6$ of original.

Reduction = $1 - 5/6 = 1/6$

In percentage:
$1/6 \times 100 = 16\frac{2}{3}%$