When students first learn percents it’s difficult for them to understand the concept. Using mental math is a great way to get them used to the idea of what percents are all about before they have to tackle much more complicated percent problems.
Remind them that
50% = 50/100 = 1/2
25% = 25/100 = 1/4
20% = 20/100 = 1/5
Bring a group of 8 students up to the front of the class. Create a scenario that the class can relate to. For example, let’s say 50% of these students love chocolate ice cream. How many students of this group love chocolate ice cream? What about if 25% of the entire group likes strawberry ice cream? How many students like strawberry ice cream?
Pick numbers of students or numbers of objects that are easily divisible to begin with so that students get used to converting 25% to 1/4 of a number or division by 4 with relative ease.
Now go backwards. Let’s say that a group of students responded to a survey about their favorite ice cream flavor. Ten of the students chose vanilla ice cream as their favorite. This number represents 20% of the group surveyed. How many students were surveyed? (50 students)
Think about having the students survey several classes of students or create an online survey using Survey Monkey to do their own ice cream flavor survey. Depending on the number of students surveyed they may need some help with their percent calculations.
Once students have a basic knowledge of percents it’s a fun activity to have them think about percents using mental math.
At the beginning start them out with some simple problems.
What is 50% of 100? (50)
What is 25% of 100? (25)
What is 20% of 100? (20)
Students should be able to understand that 50% of a number is the same as 1/2 of that number; 25% corresponds to 1/4 of a number; and 20% corresponds to 1/5 of that number. You may want to remind them that percents are based on hundredths, so 20/100 is the same as 20%. It’s also the same as 1/5 of 100 because 20/100 is equivalent to 1/5.
Now you want them to translate that knowledge to finding percents of other numbers. Of course, since they’re doing this mentally you’ll want to choose numbers that can easily be divided by 2, 4, and 5. Another option is to choose numbers where at least two of these percents will be easy for students to compute mentally or use other percents that are easier for the number in question.
What is 50% of 80?
What is 25% of 80?
What is 20% of 80?
What is 50% of 30?
What is 20% of 30?
What is 10% of 30?