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Lesson 10 – 2: Finding a Percent of a Number Mentally

Lesson 10 – 2: Finding a Percent of a Number Mentally . Objective: We will learn to use estimation and mental math to find the percent of a number.

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Lesson 10 – 2: Finding a Percent of a Number Mentally

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  1. Lesson 10 – 2: Finding a Percent of a Number Mentally Objective: We will learn to use estimation and mental math to find the percent of a number.

  2. 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.

  3. 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.

  4. 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 =

  5. Homework: Find 50%, 10% and 1% of each number. (3 checks each) • 27,000 • 5,800 • 120 • 244 • 73 • Use mental math to find each percent. (1 check each) • 25% of 200 • 10% of 40 • 5% of 700 • 15% of $8.00 • 20% of 80

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