As one values increases, so does the other.

: Find the value of 1 unit,
Multiply to find the required units.

Example

4 books cost £ 1.28. How much do 7 books cost ?

Other methods:

4:1.28  :: 7: x

This is really just the same as finding a
 scale factor for the cost of the books:-

As one values increases, the other decreases.

Example

If  it takes 4 days for 10 men to dig a trench,
 how long will it take 8 men ?

the first by the first and the last by the last

4:10  :: 8: x

Since this is indirect proportion,
the inverse applies.

© Alexander Forrest