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.
For direct proportion:
The data must have the same units
Find x given A, B and a.
If £4 is exchanged for $8.28, How many dollars will £15 be exchanged for ?
A house has appreciated by 10%.
If it is now valued at £121,000, what was the original value ?
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:
For indirect proportion, go backwards:
If it takes 4 days for 10 men to dig a trench,
how long will it take 8 men ?