Proportionality between two magnitudes

We say that there is a direct proportionality between two magnitudes if, when we multiply (or divide) one magnitude by a number, the other is multiplied (or divided) by the same number.

Examples:

- The number of sweets that I buy and the price.

- The length of the side of a square and its perimeter.

- But the measure of the side of a square and its area is not a direct proportionality.

We say that there is an inverse proportionality between two magnitudes if, when we multiply (or divide) one magnitude by a number, the other is divided (or multiplied) by the same number.

Examples:

- The number of painters that are painting a house and the time they take to do it.

- In a trip, the speed of the car and the time that it takes to get to the destination.

Exercise: decide if these pairs of magnitudes have a direct or an inverse porportionality relationship between them:

a) The height of a person and his/her shoe size.

b) The number of cheese puffs bags and the price.

c) The number of workers that are building a wall and the time that it takes them to do it.

d) The number of trucks and the amount of sand that they can carry.

Solutions: a) None of them; b) Direct; c) Inverse; d) Direct