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A method of finding a number in the same ratio to a given number as exists between two other given numbers.

‘The mathematics involves a study of proportion and percentages and introduces the rule of three for solving proportion problems.’

‘The rule of three is the familiar way of solving problems of this type: if a man earns 50 dinaras in 8 days how much will he earn in 12 days?’

‘If the fruit diminish as the requisition increases, or augment as that decreases, they, who are skilled in accounts, consider the rule of three to be inverted.’