If 3/5 of the students said they attended, then 60% attend theater class. Therefore, 2/5 or 40% of the students do not attend.
You can find this answer one of two ways. You can take 100 (percentage) and divide it by the number of total students (5), which would give you 20; therefore each student represents 20%.
Or
You can take 5 and divide that by 2, which would give you .40. You then take .40 and multiply by 100 (total percentage) and that gives your 40%.
