200 is the original sales volume, X is the price increase, and 0.5 and 10 are the original sales volume decrease 10 in 0.5 yuan for each price increase.

You can know how many times X/0.5 is the price increase, and X/0.5× 10 is the sales reduction.

As can be seen from the above, 200-X/0.5× 10 is the sales volume after reducing the purchase volume.