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Week 4

Week 4. Shopping: You have a strict budget when you are shopping for groceries. If you know that you can’t spend more than $20, how do you keep track of “how much is in your cart” so that you don’t have to put anything back?

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Week 4

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  1. Week 4 Shopping: You have a strict budget when you are shopping for groceries. If you know that you can’t spend more than $20, how do you keep track of “how much is in your cart” so that you don’t have to put anything back? You need to spend at least $10 to get a discount on school supplies. How do you keep track of how much you’ve spent in order to get the discount?

  2. Agenda • Don’t forget to initial attendance sheet! Homework “key”: a = answer with work, j = justify, s = strategy, e = extend your thinking Homework comment Estimation and Rounding Subtraction

  3. #6, 10Section 3.1 in textbook • Ask yourself… • “Were students applying correct “rules” incorrectly or do they not seem to be using the algorithm at all?” • Are the students not understanding place value or not understanding the algorithm? Is it something else? • “Would students have made any of these mistakes if they were using base 10 blocks?”

  4. Ask yourself… • “If students thought of each number as more than a collection of digits, (e.g., 56 is 5 tens and 6 ones) would these mistakes be made?” • “Do my explanations focus on rules or on the reason why, mathematically, these errors do not produce the correct answer?”

  5. Estimation • Get a QUICK answer that is close but not exact!

  6. Rounding • When you estimate, you are rounding to the nearest ten, hundred, million, etc. • If you round up, you are making an overestimation. • If you round down, you are making an underestimation.

  7. You will have 10 seconds • 382 - 147

  8. Estimation • 382 - 147 • Actual answer: 235 • Ways to estimate: • 380 - 150 = 230 • 400 - 150 - 20 = 230 • 375 - 150 = 225 • 385 - 145 = 240 • 390 - 140 = 250

  9. You try: • 45 + 87 • 391 + 1472

  10. Estimation vs. Exact Answer • Are the exact values of the addends given? • Is the exact value of the sum required? • In general: changing values (like population), measurement values (like distance), and predictions (like 35% chance of rain) are estimates

  11. Explain why the child made the mistake • 341 + 287 is the same as 340 + 290 + 7 + 3 • 749 + 684 is the same as 700 + 600 + 49 + 1 + 84 + 16 • 322 + 468 is the same as 320 + 460 • Would the mistake have been made if manipulatives had been used? Explain.

  12. Subtraction • Vocabulary: • A – B = CA is the minuendB is the subtrahendC is the difference

  13. Try this: • Is A - B = B - A? • 6 - 4 = 4 - 6? • What is true of 6 - 4 and 4 - 6? • One of the worst things we can do is lie to kids: “You can’t take a bigger number away from a smaller number.” Or “You have to put the bigger number first (in the minuend).”

  14. How do we illustrate 4-6?

  15. Pictorial Models • Take away • Judy has 7 crosses and gives 2 to Jake. How many does Judy have left?

  16. Pictorial Models • Comparison • Amy has 7 crosses and Andy has 2. How many more crosses than Andy does Amy have?

  17. Homework • Due Friday: Read Section 3.2 • Due Wednesday: • Section 3.2 p. 166-168#1abc, 4, 7cdf, 20, 28ac

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