Inverse Proportion

Inverse Proportion


For Inverse Proportion, as one value increases, the other value falls. The change in one must be at the same rate as the change in the other: if one amount doubles, then the other amount must be halved.

A typical example would involve the time taken to complete a task, and the number of people involved in the task. If the number of people on the task increased by a factor of three, then the amount of time taken to complete the task would be a third of the original time.

Multiplying one side of the proportion by a factor means multiplying the other side by the inverse of that factor.


1. An exhibition is being prepared. It normally takes 12 people six days to prepare the exhibition hall. However, only 8 people are available. How long will it take to prepare the hall?

Answer: 9 days

12 people take six days Because this is an inverse proportion (fewer people will take longer):

8 people is `frac(2)(3)` of 12

Multiply 6 days by the inverse of `frac(3)(2)`

= 6 x `frac(3)(2)` = 9

2. A car takes 5 hours for a journey at an average speed of 40mph. How long will the journey take if the average speed is 55mph? Give the answer to the nearest minute.

Answer: 3hr 38m

This is an inverse proportion; the higher the value of the speed, the smaller the value of the time.

40mph takes 60 x 5 = 300 minutes

55 mph is `frac(55)(40)` of 40. Simplify to `frac(11)(8)`

Multiply 300 minutes by the inverse = 300 x `frac(8)(11)` = 218.18 minutes

This is three hours and 38 minutes (nearest minute)