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- 1. PERCENTS HANDOUT #1 - NOTES MATH 015 Use the following for problems 1 – 10: TO CHANGE A DECIMAL TO A PERCENT: Multiply by 100%. Example: 0.005 = 0.005 100% = 0.5% TO CHANGE FROM A PERCENT TO A DECIMAL: Divide by 100%. Example: 4.875% = 4.875% 100% = 0.04875 TO CHANGE A FRACTION TO A PERCENT: Multiply the fraction by 100%. Simplify. 1 1 100 Example: 100% % 25% 4 4 4 TO CHANGE FROM A PERCENT TO A FRACTION: Divide the percent by 100%. Then simplify. 5 1 Example: 0.5% = 0.5% 100% = .005 = 1000 200 TO CHANGE FROM A DECIMAL (OR FRACTION) PERCENT TO A FRACTION: Divide by 100%. Then simplify. 14 7 Example: 1.4% 1.4% 100% 0.014 1000 500 2 2 2 1 1 Example: % % 100% g 5 5 5 100 250 1
- 2. MENTAL COMPUTATIONS INVOLVING COMMON PERCENTS Use the following for problems 11 – 20. 1 Fifty Percent The fraction form of 50% is , so verbally 50% means “half”. Finding 50% 2 of a number means finding “half” of that number. Remember that to find half of a number is the same as dividing by two. Example 1: What is 50% of 48? 50% of 48 means half of 48, which is 24. Answer: 24 is 50% of 48. One Hundred 100% is equivalent to the whole number 1. 100% means the whole or Percent “all”, so to take 100% of a number means to take “all” of the number. Example 2: 100% of what number is 3.6? All of the unknown number is 3.6, so the unknown number must be 3.6. Answer: 100% of 3.6 is 3.6. Two Hundred 200% is equivalent to the whole number 2. 200% means “double”, so Percent to find 200% means to “double” the number. Remember that to double a number is to multiply the number by 2. Example 3: What is 200% of 30? To find 200% of 30 means to double 30, so the answer is 60. Answer: 60 is 200% of 30. Example 4: 30 is 200% of what number? 2
- 3. This problem is different from the previous example. Here, we know that 30 is double the unknown number, so the unknown number must be half of 30. The answer is 15. Answer: 30 is 200% of 15. 1 Ten Percent 10% is equivalent to the fraction and the decimal 0.1. To find 10% of a 10 1 1 number means to find of the number or multiply the number by . 10 10 1 Remember, though, that multiplying by is the same as dividing by 10 and 10 that to divide a number by 10, move the decimal point one place to the left. Therefore, to find 10% of a number means to divide the number by 10 by moving its decimal point one place to the left. Example 5: 10% of $43.50 is what number? To find 10% of $43.50, divide $43.50 by 10 (move the decimal point one place to the left). Answer: 10% of $43.50 is $4.35. 1 Twenty-five 25% is equivalent to the fraction or the decimal 0.25. To find 25% 4 1 1 Percent of a number means to find of the number or to multiply the number by . 4 4 1 Remember that multiplying a number by is the same as dividing the number 4 by 4. Therefore, to find 25% of a number is to divide the number by 4. Example 6: What is 25% of 2000? To find 25% of 2000 means to divide 2000 by 4, so the answer is 500. Answer: 500 is 25% of 2000. Observe You may have noticed that 25% is half of 50%. Another way of finding 25% of a number is to find 50% (half) and then take half of that answer. It is sometimes quicker to do these two calculations mentally than to try to divide by 4. In this example, 50% of 2000 is 1000, and half of 1000 is 500. Example 7: 20 is 25% of what number? 3
- 4. 1 20 is of the unknown number, so the unknown number must be 4 times 20, 4 or 80. Answer: 20 is 25% of 80. 4

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