Inverse proportion is when two quantities change opposite to one another.

As one quantity increases, the other quantity decreases.
When the two quantities are multiplied, the same constant occurs.

A common example of inverse proportion is to calculate the time it takes to do a job with more or fewer workers.

Example One - Painting a Wall Mural

3 talented graffiti artists can paint a wall mural in 10 hours. To paint the same mural on another wall, calculate the time it would take:
(a) 1 artist
(b) 6 artists.

(b) Time with 5 men
= 80 man-hours ÷ 5 men
= 16 hours

Question - Building a Fence

It takes 6 people 8 hours to build a fence around a farm. How long would the same fence have taken to build with 12 people?
(Hint: Find the number of hours with only 1 person first.)

Answer 4 hours

Maths Fun - BMI (Body Mass Index)

Doctors use a Body Mass Index to determine if people are healthy or not. A BMI of 25 is healthy. Less than 25 is underweight. More than 25 is overweight.

Body Mass Index =

Mass in kg

Height in metres-squared

Measure your mass in kilograms. Measure your height in metres. Calculate your BMI.
According to the rule above, are you healthy? What problems are there with this rule?