As one values increases, so does the other.
: Find the value of 1 unit,
Multiply to find the required units.
4 books cost £ 1.28. How much do 7 books cost ?
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.
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.