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The VAT rate is at present 17·5%.
To calculate the VAT on, say, £150, we need to work out (17·5 x 150)/100 = £26·50.
We could also do it as 17·5 x 1·5 or as 17 ½ x 1 ½.
Then again, if we notice that 17·5 = 10 + 5 + 2·5, we can simplify the working for pencil and paper as
| 10% of 150 | is £15·00 |
| 5% is half 10% | or £7·50 |
| 2·5% is half of 5% | or £3·75 |
| add these results | to get £26·25 |
This method was usually called "Practice" and is very useful for calculations in Imperial units, (or pre-metric units in other countries).
2·5% is also sixpence (1) in the pound, so 17·5% is 7 sixpences in the pound, and we could, if we wanted to, calculate the answer as 7 x 150 sixpences = 7 x 75s = 525s = £26.5s.0d.
Sixpence is a fortieth of a pound, so what we are really doing is expressing the fraction 7/40 as 1/10 + 1/20 + 1/40, which is what the Ancient Egyptians would have done.
So are the methods found in the Egyptian scrolls really recipes for doing sums by Practice methods?
(1) 6d, not 6p...