Predict the result
Write twenty four over seventy two in simplest form.
Which positive whole-number factor divides both parts?

Set up the model
Use the same nonzero common factor for the complete numerator and denominator.
Eight is a common factor of twenty four and seventy two.

Work the example
Divide both complete parts by eight. The fraction becomes three ninths.
Three and nine still have a common factor greater than one.
Divide both complete parts by three. The fraction becomes one third.
One and three have no common positive whole-number factor greater than one.

Check the result
Now write twenty eight over forty two in simplest form.
Use the same nonzero common factor. Take eight seconds to think.
[Eight-second silent pause to work; the answer remains hidden.]
The greatest common factor of twenty eight and forty two is fourteen.
Divide both complete parts by fourteen. The simplest form is two thirds.

Interpret the result
Dividing both complete parts by the same nonzero common factor keeps the fraction equal.
For these positive whole-number parts, stop when no common factor greater than one remains.