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.