The choice when expressing numbers is between using a word to express the number (e.g., three) or a figure (e.g., 3).  Please check whether or not the number is expressed correctly.  Then  highlight the rule that governs the use of the number in this instance.

A math test of 20 simple arithmetic problems developed by the experimenter was used for a distraction task.

The expression of the number is   correct     incorrect       because: