Unit price * quantity = turnover

The sales volume has increased by 1/5, and the turnover should be 1+ 1/5=6/5 at the original price.

Volume decreased by 2/5 to 1-2/5=3/5.

The unit price is the original price: (3/5)/(6/5)= 1/2=0.5.

So 50% off.