How to Do Basic Percentages with the Table of Joy
The Table of Joy is a technique for figuring out what sum you need to do when you have two amounts you know to be proportional – that is, if you double the size of one, you double the size of the other. This includes things like percentages and ratios, as well as dozens of other topics, from currency conversion to cake recipes and beyond.
To show you how it’s done, work out 20% of £340.
Drawing a table, so you have a framework to work in.
The Table of Joy looks like an oversized noughts and crosses grid. Make sure you have space to fit labels in the rows and columns.
The top row and the first column are for labels, so that you know which number to put where; the four squares in the bottom right are for writing the numbers in the sum.
Labelling the rows and columns, so you know what goes where.
You may be tempted to skip this step, but it’s actually quite important. By labelling your rows and columns, you make it a piece of cake to figure out which number has to go where.
Filling in the gaps with the information you’ve been given.
Now it’s time to fill in the numbers. The whole of anything is always 100% – in almost every percentage sum you do, you’ll need to put 100% in one of the boxes. Here, it goes in the middle right – in the ‘per cent’ column and the ‘whole thing’ row.
The question also states that the ‘whole thing’ is £340, so that has to go in the ‘whole thing’ row and the ‘pounds’ column – the middle.
The last number you know about is that your answer has to be 20% of the whole thing, so you have to put 20 in the ‘per cent’ column and the ‘part’ row, which is the bottom right square.
Working out the sum, which is normally the easy bit.
Shade in the grid like a chessboard, so the squares alternate between dark and light, with the shaded squares making an ‘X’.
Find the two numbers that are in the same coloured squares and write them down with a x between them (in this case, 340 x 20).
After this, write down a divide sign and the remaining number. For this example, the Table of Joy sum is 340 x 20 ÷ 100.
Do the sum 340 x 20 = 6,800; divide this by 100 to get 68 – which means £68 is 20% of £340.