Two quantities are said to be in direct proportion if they increase or decrease in the same ratio. If two amounts are directly proportional we can scale the quantities up by multiplying. 600 + 150 = ...

There is a direct proportion between two values when one is a multiple of the other. For example, \(1\text{cm} = 10\text{mm}.\) To convert \(\text{cm}\) to \(\text{mm}\), the multiplier is always 10.

Twenty secondary mathematics teachers described their thinking as they solved four proportion problems (Task 1), explained how they would teach two of the problems (Task 2), and solved four similar ...