Finding a Percent of a Number Mentally

Objective:

We will learn to use estimation and mental math to find the percent of a number.

It’s your best friends birthday and you invite him and a group of friends to Chili’s. The restaurant bill was $42 dollars. The waiter treated you really well and you want to give him an extra 10% percent tip. How much money will you pay the waiter??

Answer: To make a quick calculation mentally, all you have to do is move the decimal point one place to the left.

$42. = 4.2 = $4.20 is 10% which is what you should give the waiter.

Rule #1: 10% of a number is 1 of a number. 10 You can solve it mentally by moving the decimal

point to the left 1 space.

Example: 10% of 270 = 27. 0 or 27.

• Rule #2: 1% of a number is 1 of a number. 100• You can solve it mentally by moving the decimal point 2 places to the left.

• Example: 1% of 270 = 2.70. or 2.7

• Rule #3: 50% of a number is 1 of the number, which means half times the number. 2

•Example: 50% of 270 = 135

• You can solve it mentally by taking the number and dividing it by 2.

When you are not sure about the percent of a number, all you have to do is write the percent as a decimal and multiply by the number in the problem.

Example: 25% of 500

.25 x 500 = 125

Your Example:

50% of 6= 5% of 300 =

20% of 80 = 90% of 500 =

Homework: Find 50%, 10% and 1% of each number. (3 checks each)

1. 27,000

2. 5,800

3. 120

4. 244

5. 73

Use mental math to find each percent. (1 check each)

6. 25% of 200

7. 10% of 40

8. 5% of 700

9. 15% of $8.00

10. 20% of 80