When entrepreneurs in the Dragon's den ask for say £100,000 in return for 5% of their businesses, the dragons would have to do a quick mental maths calculation to work out how much the business is being valued at.  In simple 11+ maths style, the question would be:

If 5% of a number is £100,000, how much is the number?   A quick solution is to divide 100 by the percentage and multiply the number by this answer.  So in this case 100/5 = 20, so the entrepreneur thinks his business is worth 20 x £100,000 or £2,000,000.

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