Answer:
20% of the students in Dave's class don't have a cell phone.
Explanation:
Multiply 4/5 by 100:

Cross simplify 4/5 and 100 to change 5 to 1 and 100 to 20:

Once you multiply, you should get 80%. 80% is the number of students that have a cell phone. To get the number of students who don't have a cell phone, we have to subtract 100% by 80%:
100%
-80%
--------
20%
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