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Multiplying Decimals

Multiplying Decimals. Lesson 3.4. Using Base Ten Blocks. The flat represents 1, the rod represents 0.1 and the small cube represents 0.01 By using Base Ten Blocks to make a rectangle, you can switch to addition instead of multiplication for decimals. Modeling.

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Multiplying Decimals

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  1. Multiplying Decimals Lesson 3.4

  2. Using Base Ten Blocks • The flat represents 1, the rod represents 0.1 and the small cube represents 0.01 • By using Base Ten Blocks to make a rectangle, you can switch to addition instead of multiplication for decimals.

  3. Modeling • A rectangular park measures 1.7 km by 2.5 km. Step 1: Build a rectangle with length of 2.5 and width 1.7 Width (1.7 km) When the rectangle is finished, count the blocks in the rectangle. Length (2.5 km)

  4. There should be 2 flats: 2 x 1 = 2 • There should be 19 rods: 19 x 0.1 = 1.9 • There should be 35 cubes: 35 x 0.01 = .35 • The total area will be: 2 + 1.9 + .35 = _________

  5. But How Do I Multiply?? • 1.7 x 2.5 • Pretend there are no decimals. • 17 x 25 • Multiply as normal. • Then add the decimal after.

  6. But Where Do I Put The Decimal? 1.7 x 2.5 1.7 x 2.5 4.25 Ask yourself, “How many numbers are after the decimal?” = 1 Ask yourself, “How many numbers are after the decimal? = 1 There are 2 numbers total after the decimal point. Start at the end of the number, then, move two numbers to the left. Where you stop the decimal goes.

  7. How Do I Multiply 2 Two Digit Numbers? • A brief reminder... 13 x 12 26 190 216 1) Multiply 3 x 2 first. Put the answer below the 2. (6) 2) Then, multiply 2 x 1. Put the answer beside the 2 and under then 1. (2) 3) Then, put a zero under the 2. (0) 4) Then multiply 1 x 9. Put the answer beside the zero and under the 1. (9) 5) Then, multiply 1 x 1. Put this answer beside the 9. 6) Add the two numbers together to get your answer.

  8. Try these with long multiplication. • 14 x 21 • 17 x 12 • 15 x 20

  9. Word Problems • At the Farmers Market, 1 kg of grapes costs $2.95. How much would 1.8 kg of grapes cost? • 1 kg of grapes costs $2.95. • So, 1.8 kg would cost 2.95 x 1.8 Create a rectangle model (Base Ten)

  10. 2.95 x 1.8 • (2.0 x 1.0) + (0.95 x 1.0) + (2.0 x 0.8) + (0.95 x 0.8) • (2 x 1) + (0.95) + (2 x 0.8) +(0.95 x 0.8) • 2 + 0.95 + 1.6 + 0.76 = 5.31 Therefore, 1.8 kg of grapes would cost $5.31. 2.95 2.0 x 1.0 0.95 x 1.0 1.8 2.0 x 0.8 0.95 x 0.8

  11. A kg of apples costs $1.90. • How much would 2.1 kg of apples cost? • Draw the rectangle model and solve.

  12. An area rug is rectangular. Its dimensions are 3.4 m by 2.7 m. • Solve using multiplication.

  13. Estimation • To estimate using multiplication, multiply the two whole numbers together. (First round up or down to the nearest whole number) • 5.3 x 2.1 = 5 x 2 = 10 • A good estimation is 10. • Multiply to verify your estimation.

  14. Use estimation, then solve using multiplication. Remember to round up or down to the nearest whole number) • 2.6 x 1.3 • 7.2 x 4.8 • 3.9 x 3.2

  15. Homework • Workbook 3.4 • Textbook Page 102 – 103 # 4, 5, 9, 10 and Reflect

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