Directly proportional quantities

A furnace has its cast iron production according to the table below:

Time (minutes)Production (Kg)

Note that one quantity varies with the other. These quantities are dependent variables. Notice that:

When we duplicate time, production also doubles.
5 min -> 100Kg
10 min -> 200Kg

When we tripled time, production also triples.
5 min -> 100Kg
15 min -> 300Kg

Like this:

Two dependent variable quantities are directly proportional when the ratio between the values ​​of the 1st quantity is equal to the ratio between the corresponding values ​​of the 2nd

Verify in the table that the ratio between two values ​​of one quantity is equal to the ratio between the two corresponding values ​​of the other quantity.

Next: Inverse Proportional Quantities