Setup
A proportion states a/b = c/d: the two ratios are equal. If one value is missing and no denominator is zero, cross multiply: ad = bc, then solve.
Example
If 2 notebooks cost $5, what do 5 notebooks cost? Match notebooks to dollars on both sides, e.g. 2/$5 = 5/x (or 2/5 = 5/x only if you keep the same units top and bottom). Wrong pairing gives wrong x.
Check
Mistakes
- Matching numerator to the wrong kind of quantity.
- Forgetting units in the answer.
Use the calculator
FAQ
- Cross multiply always?
For simple proportions with nonzero denominators, yes. Understanding the setup still matters.