Estimating percentages. According to google
If we are trying to find n percent of x, we can estimate this percent using the following steps:
Round both n and x up or down to numbers that are easy to work with.
Multiply the rounded numbers together.
Divide the result by 100.
(£1000 x £3000) / 100 = 30
so it would seem that this increase is 30% I am not happy with this but do not have time to work out the reason.
However wikipedia says
Percentage increase and decrease[edit]
Due to inconsistent usage, it is not always clear from the context what a percentage is relative to. When speaking of a "10% rise" or a "10% fall" in a quantity, the usual interpretation is that this is relative to the initial value of that quantity. For example, if an item is initially priced at $200 and the price rises 10% (an increase of $20), the new price will be $220. Note that this final price is 110% of the initial price (100% + 10% = 110%).
Some other examples of percent changes:
An increase of 100% in a quantity means that the final amount is 200% of the initial amount (100% of initial + 100% of increase = 200% of initial); in other words, the quantity has doubled.
An increase of 800% means the final amount is 9 times the original (100% + 800% = 900% = 9 times as large).
A decrease of 60% means the final amount is 40% of the original (100% – 60% = 40%).
A decrease of 100% means the final amount is zero (100% – 100% = 0%).
In general, a change of x percent in a quantity results in a final amount that is 100 + x percent of the original amount (equivalently, 1 + 0.01x times the original amount).
Clearly all very interesting. I would consider however that percentages are to do with an increase in fractions of the whole and that when the price triples we just say so. And all this goes to show how easy it is to misuse arithmetic.