Students will develop an understanding of the rules of rounding to the nearest ten.
Class: 3rd Grade
Duration: One 45 minute class period
Key Vocabulary: estimate, rounding, nearest ten
Objectives: Students will understand simple situations in which to round up to the next ten or down to the previous ten.
Standards Met: 3.NBT. Use place value understanding and properties of operations to perform multi-digit arithmetic. Use place value understanding to round whole numbers to the nearest 10 or 100.
Lesson Introduction: Present this question to the class: The gum Sheila wanted to buy costs $.26. Should she give the cashier $.20 or $.30? Have students discuss answers to this question in pairs, and then as a whole class.
After some discussion, introduce 22 + 34 + 19 + 81 to the class. How difficult is this to do in your head? (Give them some time, and be sure to reward the kids who get the answer, or who get close to an exact answer) If we changed it to be 20 + 30 + 20 + 80, is that easier?
Step-by Step Procedure:
- Introduce the lesson target to students: Today, we will be introducing the rules of rounding. Define rounding for the students. Discuss why rounding and estimation is so important. Later in the year, we will get into situations that don’t actually follow these rules, but they are important to learn in the meantime!
- Draw a simple hill on the blackboard. Write the numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 so that the one and 10 are at the bottom of the hill, on opposite sides, and the five ends up at the very top of the hill. This hill will be used to illustrate the two tens that the students are choosing between when they are rounding.
- Tell students that today we will focus on two-digit numbers. They have two choices with a problem like Sheila’s. She could have given the cashier two dimes ($0.20) or three dimes ($0.30). What she is doing when she figures out the answer is called rounding - finding the closest “10” to the actual number.
- With a number like 29, this is easy. We can easily see that 29 is very close to 30. But with numbers like 24, 25, and 26, it gets a little more difficult. That’s where our mental hill comes in.
- Ask students to pretend that they are on a bike. If they ride it up to the 4 (as in 24), and stop, where is the bike most likely to head? (Back down to where they started.) So when you have a number like 24, and you are asked to round it to the nearest 10, the nearest 10 is backwards, which gets you right back to 20.
- Continue to do the hill problems with the following numbers. Model for the first three with student input, then continue with guided practice, or have students do the last three in pairs:
- What should we do with a number like 35? Discuss this as a class, and refer back to Sheila’s problem at the beginning. The “rule” is that we round to the next highest 10, even though the five is exactly in the middle.
Homework/Assessment: have students do six problems like the ones in class. Offer an extension for students who are already doing well, to round the following numbers to the nearest 10:
Evaluation: At the end of the lesson, give students an exit card with three rounding problems of your choice. You will want to wait and see how students are faring with this topic before choosing the complexity of the problems you'll give them for this assessment. Use the answers on this exit card to group students and provide differentiated instruction during the next rounding class period.