Rule of three: finding a number from its fractional part
Two methods that both give 12 3/5 as the number whose 5/9 is 7
A short arithmetic exercise, written upside down and mostly on the lower part of an otherwise blank page. Leonardo asks for the number of which 7 is 5/9, solving it first by the rule of three ('if 5 were 9, what would 7 be?') and then by a direct method of dividing 7 by 5 and multiplying by 9, both giving 12 and 3/5.
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Number whose 5/9 equals 7, by two routes
By the rule of three: if 5 corresponded to 9, then 7 corresponds to 12 and 3/5. The alternative rule divides 7 by 5 and multiplies the quotient by 9, reaching the same 12 and 3/5.