You can calculate the original amount from before a percentage rise/fall by reversing the change.
Part of MathsNumerical skills
A shop offers 30% off everything in a sale. The sale price of a pair of designer shoes is ÂŁ84. Calculate the cost of the shoes before the sale.
\(Sale\,of\,30\% \to (100\% - 30\% = 70\% )\)
70% of the cost of the shoes = ÂŁ84
\(1\% \to 84 \div 70 = \pounds1.20\)
\(100\% \to \pounds1.20 \times 100 = \pounds120\)
The cost of the shoes before the sale was ÂŁ120.
Now try the example question below.
After a 4% wage rise, Bill earned ÂŁ1248 each month.
Calculate how much Bill earned each month before the wage rise.
\(4\% \,rise\\), therefore:
\(100\% + 4\% = 104\%\)
\(104\% = \pounds1248\)
\(1\% = \pounds1248 \div 104\)
\(= \pounds12\)
\(100\% = \pounds12 \times 100\)
\(= \pounds1200\)
Bill earned ÂŁ1200 each month before the wage rise.