Composed Rule of Three

Composed Rule of Three is an application for the daily life, which is not always at hand everything you need to perform simple operations like:

"If 3 orange cost 9 pounds, how much do 20 oranges cost?"

Perform Rules of three (Cross multiplication), simple or compound, direct or inverse, very quick and easy. As we did in school, and very useful to reinforce concepts and test results for typical exercises:

"A cow gives me two litres of milk in 10 hours, how many hours did it take me 3 cows in 30 litres of milk?"

"A worker takes 3 hours to build a wall, how many workers are needed to build two walls in five hours?"

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