This document provides examples of using two methods to estimate sums and differences of decimals: 1) Rounding decimals to the nearest whole number and then adding or subtracting 2) Using front-end estimation, where you add the front digits and then adjust based on the remaining digits It demonstrates both methods on multiple examples and asks students to practice estimating sums and differences to determine if they have enough money to purchase several items.

1. When is estimation better than counting and when not?

2. Learning Target:Students will learn how to estimate sums and differences of decimals.

3. What were the rule(s) we learned for estimating sums and differences of whole numbers?

4. When estimating sums and differences of decimals… One way is to round the decimal to the nearest whole number and then add or subtract.(Whole number is the first place to the left of the decimal point.)

5. Use rounding to estimate the sum or difference.1. 27.2 – 12.72. 11.25 + 8.86

6. Answers:1. 27.2 – 12.727 – 13142. 11.25 + 8.8611 + 920

7. The second way you can estimate the sum is using front-end estimation. You add the front-end digits to get a low estimate. Then you use the remaining digits to adjust the sum and get a closer estimate.

8. Use front-end estimation to estimate the sum.6.42 + 7.64 + 3.94 + 2.21

9. 6.42 + 7.64 + 3.94 + 2.21add the front-end digits6 + 7 + 3 + 2 = 18use the remaining digits.42 + .64 = 1 and .94 + .21 = 1adjust the sum for a closer estimate18 + 1 + 1 = 20

10. Use front-end estimation to estimate the sum.8.59 + 1.37 + 2 + 6.12

11. Answers:8.59 + 1.37 + 2 + 6.128 + 1 + 2 + 6 = 17.59 + .37 + .12 = 117 + 1 = 18

12. Can you buy all the items listed with $20?Notebook $3.85Paper $2.29Calculator $8.39Mouse Pad $2.65Which choice of estimation would you use?

13. Answers:1. Round to whole number4 + 2 + 8 + 3 = 1717 is less than 202. Front-end estimation (best choice)3 + 2 + 8 + 2 = 15.39 + .65 = 1.85 + .29 = 115 + 1 + 1 = 1717 is less than 20Yes, you could buy all the items with $20.

14. Predicting ResultsYou buy a ticket for $22.85 with $30. Estimate your change. Is the estimate high or low?

15. Answer:22.85 rounds to 2330 – 23 = 7Your change is about $7 which would be a low estimate because you will really get back a little more then $7.

16. Can you estimate sums and differences of decimals?