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Students learn to estimate the sum up to 100 by rounding to tens.
Start by setting three numbers (to 100) on the number line and decide what their closest tens number is. Then round the numbers to either 40 or 50. Ask students to explain how they know that a given number is rounded to 40 or 50.
Explain that when you estimate a sum, you first round the addends to their nearest ten, and then you add them together. That means that 19+42 is calculated as 20+40. Practice estimating the sum rounding in tens by first making the estimated math problem, and then solving for the total. Do a variety of these exercises together with the students.
Next, you practice examples taken from daily life in which you estimate totals. Using money, you ask if there is enough money to buy the soccer supplies. Practice rounding to the tens and calculating the estimated total.
To check that students understand estimating the total to 100 you can ask the following questions:
- Why is it useful to be able to estimate the sum?
- What do you do with the addends when estimating addition?
Students first practice estimating the total by rounding the addends to the nearest tens. Then students are asked to answer the question of "Is _ enough" in a money context, and finally a story problem in which they discover about how many there are.
Discuss with students why it is useful to be able to estimate the sum. It is useful if you want to know about how many you still need of something. You can also use it when calculating the exact amount of something because you already know how much it should be. If your exact answer is close, you are very likely correct. To close, show students some statements where students must decide if estimating the sum is useful or not (for example in the supermarket). You can have the students form groups or pairs and ask them to think about the statements. To close and check that students are able to estimate the sum ask students to solve three addition problems where they must select the correct estimated answer.
Students who have difficulty with estimating the sum can first practice rounding numbers to their nearest tens. Make sure that students who do these sums slowly are not solving the exact problem and then rounding the total. Make sure that they round the addends, before finding the total.
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