Suppose the cost last year was X, then the cost this year is X (1-20%).

Suppose the price last year was Y, then the price this year is Y (1- 10%).

Suppose last year's sales volume was m and this year's sales volume was n.

By: The profit per unit increased by 15 percentage points compared with last year.

Profit = selling price-cost

∴(y-x)( 1+ 15%)=y( 1- 10%)-x( 1-20%)①

Z: The total profit this year is the same as last year.

∫ Profit = (selling price-cost) × sales volume

∴(y-x)m=[y( 1- 10%)-x( 1-20%)]n②

Through ① and ②

Solve n/m=20/23

The ratio of this year's sales to last year's sales is 20 to 23.