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Multiplying Whole Numbers and Decimals

Multiplying Whole Numbers and Decimals. Unit 3 Lesson 8 and 9 Powerpoint taken from: http://www.mrsoregan.net/PwrPnt_Notes.htm. Things you need to know…. Do not line up the decimal points! Pretend all decimals are whole numbers until the very end

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Multiplying Whole Numbers and Decimals

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  1. Multiplying Whole Numbers and Decimals Unit 3 Lesson 8 and 9 Powerpoint taken from: http://www.mrsoregan.net/PwrPnt_Notes.htm

  2. Things you need to know… • Do not line up the decimal points! • Pretend all decimals are whole numbers until the very end • Only look at the decimals when trying to figure out where to put the decimal point in your answer

  3. or Strategy #1 405 • 59 = x 405 x 59 405 x 9 405 x 50 + 3 6 4 4 5 2 0 2 2 5 0 (5 x 4) = 20 Since there are no other numbers to multiply, write down the 20. (5 x 0) + 2 = 2 Write down the 2. Zero is a place holder. Put a zero in your answer to hold its place. (9 x 0) + 4 = 4 Write down the 4. 5 x 9 = 45 Put the 5 in the answer and carry the 4 (it’s really 40) to the next place value (tens) 5 x 5 = 25 Put the 5 in the answer and carry the 2 (it’s really 20) to the next place value (tens) (9 x 4) = 36 Since there are no other numbers to multiply, write down the 36.

  4. or Strategy #1 405 • 59 = x 405 x 59 405 x 9 405 x 50 + 3 6 4 4 5 2 0 2 2 5 0 23,895 The final answer is:

  5. What if I have a decimal point in my problem? • Multiply your numbers using one of the steps just shown. • When done, count all the numbers after a decimal point in your problem. • Make sure your answer has exactly that number of digits in the decimal spaces.

  6. If my problem has decimals… 405 x 59 4.05 x 5.9 4.05 x 5.9 .405 x .059 .405 x .059 23,895 23.895 .023895 I had to add a zero because there are six total decimals spots in my problem, but I only had 5 digits in my answer. Notice that the zero came between the decimal point and my old answer.

  7. Let’s try one more…

  8. What if I have a decimal point in my problem? • Multiply your numbers using one of the steps just shown. • When done, count all the numbers after a decimal point in your problem. • Make sure your answer has exactly that number of digits in the decimal spaces.

  9. If my problem has decimals… 39.5 x 8.4 39.5 x 8.4 .395 x .084 .395 x .084 395 x 84 .033180 331.80 33180 I had to add a zero because there are six total decimals spots in my problem, but I only had 5 digits in my answer. Notice that the zero came between the decimal point and my old answer.

  10. Let’s Practice…

  11. 78.6 x 983 78.6 X 983 2358 62880 + 707400 772,638 77263.8 772638

  12. .392 x .008 1 There are 6 digits after a decimal point. .392 X .008 .003136 .003136 003136

  13. 9358 x .3 2807.4

  14. 115.6 x .004 .4624

  15. .0018 x 7 .0216

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