Composed Rule of Three description

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 8 dollars, 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 liters of milk in 10 hours, how many hours did it take me 3 cows in 30 liters 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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