Predict the result
Are three over seven and nine over twenty two equal?

Set up the model
Both denominators are nonzero.
Compare the cross products.

Work the example
Three times twenty two is sixty six; nine times seven is sixty three.
The cross products differ, so the fractions are not equal.

Check the result
Are eight eighteenths and twelve twenty sevenths equal?
Take eight seconds to think before the answer appears.
[Eight-second silent pause to work; the answer remains hidden.]
Both cross products equal two hundred sixteen, so the fractions are equal.

Interpret the result
Cross multiplication tests a proportion only when its original denominators are nonzero.