While it's easier to use the Omni Percentage Increase Calculator, here are the steps to calculate the discount rate in Excel:
Input the original number (for example, into cell A1).
Input the increased number (for example, into cell B1).
Subtract the original number from the increased number (In C1, input =B1-A1) and label it 'difference'.
Divide the difference by the original price and multiply it by 100 (In D1, input =(C1/A1)*100) and label it 'percentage increase'.
Right-click on the final cell and select Format Cells.
In the Format Cells box, under Number, select Percentage and specify your desired number of decimal places.
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