% Percentages: 'Close enough for Government work.' %
Maths Difficulties and Dyscalculia. A series of articles by Steve Chinn.
www.mathsexplained.co.uk
Percentages.
Most people can compare percentages. Probably because they can count from 0 to 100. So, they know that a 50% discount saves more money than a 25% discount on the same item. But can they work out what money they save?
I have some evidence to support my concern. The first is from my own research. 37% of my sample (360) of 16 to 19 year olds got ‘150% of ￡64’ wrong.
The second is from some recent research from New Zealand by Sharpe and Hughson, who found that a very small percentage of students from a similar age group could work out what a 20% saving on a $320 gym fee would be. Making a number problem into a word problem always lowers the number of correct answers.
So, what happens with another common use of percentages, tipping? In the UK tipping is often 10% or 12.5% (it’s higher in the USA, especially NYC).
There is a chance that many people can work out 10% of the bill, though a restaurant would be wise to show what this amount is when presenting the bill/check. But 12.5%? That’s going to cause problems. Of course, such a calculation can be done on a phone, but to work this out ‘in your head’?
It might raise some eyebrows if the diner starts writing on the tablecloth, say for a $128 bill, ‘128 x 12.5 ÷ 100’ and then, as taught in school, writes down the steps used to arrive at an answer. A good ‘life maths’ person will avoid this formula. They will already have an idea, an estimate of how much to tip will be. For example, they might work out 10% and get $12.80 and then add on a couple of dollars to take it closer to 12.5%.
If there is a knowledge of the link between percentages and fractions, they link 12.5% to 1/8, and then halve, halve and halve again, so 64, 32 and then $16. Quite often it is good to play with numbers and concepts until you find the best way, for you, to work out a particular calculation. Often, in everyday life you do not need to get a precise answer, so this gives you the freedom to use what works for you. An ex-student of mine used to say when he got his answer, ‘Close enough for Government work!’