Ok let me state that if it is reduced by 25% is the same a multiplied by 0.75. Increased by 20% is the same as multiplied by 1.20. So what you need to do is multiply the two multipliers together to get the net effect. Now let me give you an example: If I sell 100 things a day and sales go up 20% I sell 120 things a day. That is the same as multiplying sales by 1.20. So the formula you can use is
Gross Receipts = Sales Price * Number of units sold.
If I change Sales Price and Number of units sold each by simple multipliers, the effect on Gross Receipts will be the product of the multipliers. I hope this helps