The Rule of Three

 

This is used to find the value of a fourth quantity

when the relationship of two quantities is fixed and

the value of the third quantity is given.

 

·       Align the data in correct columns

·       Multiply 3^rd data by 2^nd

·       Then divide by first.

 

The data must have the same units

 

Example

 

 Find x given A, B and a.

 

     

 

 

Example

 

If £4 is exchanged for $8.28,

How many dollars will £15 be

 exchanged for ?

 

 

 

 

 

 

 

 

Example

 

A house has appreciated by 10%.

If it is now valued at £121,000,

 what was the original value ?

 

 

 

 

 

Example

 

A map has scale 3 : 50000

What is the real life distance to  a town

if the distance on the map is 5.4 cm ?

Give your answer in kilometres.

 

 

 

 

 

 

 

Why does it work ?

 

This is effectively the same as finding the scale factor

and multiplying by the corresponding side: