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

Time (minutes) | Production (Kg) |

5 | 100 |

10 | 200 |

15 | 300 |

20 | 400 |

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