What is proportion?
- Proportion is a way of talking about how two variables are related to each other
- Direct proportion: as one variable goes up the other goes up by the same factor
eg. If one variable is multiplied by 3, so is the other
- Inverse proportion: as one variable goes up the other goes down by the same factor
eg. If one variable is multiplied by 3, the other is divided by 3
How do we deal with proportion questions?
- Identify the two VARIABLES (called A and B below)
- Choose TYPE of proportion:
A is DIRECTLY proportional to B use formula A = kB
A is INVERSELY proportional to B use formula A = k÷B
- FIND k using the values given in the question
- Write FORMULA for A in terms of B (using your value of k)
- USE formula to find the required quantity
Even if the question doesn’t ask for a formula (Step 4. above) it is always worth working one out and using it in all but the simplest cases.