Percentages are usually taught by one of two methods; multipliers by decimals or the unitary method. The multipliers by decimals is the best method to use as it works in all situations. Returning to the example used in the previous section;
The cost of the reactants increased by 15%. The new price was £122.00, what was the old price?
The common mistake made here, is that students often take 15% off £122.00, which gives an incorrect answer of £96.70, instead of simply dividing by the multiplier 1.15, which gives an answer of £106.09.
The unitary method works well for scaling up, but it causes even more problems when it comes to scaling down as in the above example. The common mistake is thinking that the new price, in this case £122.00 represents 100%, and working out 1% as £1.22 rather than realising that £122 is actually 115% and so 1% is (122.00/115) 1.06.
Therefore it is recommended that the multiplier by decimals method is used.