Definition of the Direct Rule of Three
The simple direct rule of three consists of a relationship between quantities with direct proportionality, which occurs when given two corresponding quantities of directly proportional magnitudes, we must calculate the quantity of one of these magnitudes corresponding to a given quantity of the other magnitude.
We will apply the direct rule of three when the following relationships are established between the magnitudes:
More 
more.
Less 
less.
That is, when one magnitude increases, the other also increases, and if one magnitude decreases, the other does as well.
Examples of the Simple Direct Rule of Three
1. A car travels 
miles in 
hours. How many miles will it have traveled in 
hours?
These are directly proportional magnitudes, since in fewer hours it will travel fewer miles.
Solution:
mi 
h
mi h
mi
2. Ana buys 
lb of potatoes. If 
lb cost 
, how much will Ana pay?
These are directly proportional magnitudes, since more pounds means more dollars.
Solution:
lb 
¢
lb 
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