A Lesson Plan to Teach Rounding by 10s

Teaching the Concept of Rounding Numbers Up and Down by 10s

@ tpholland

In this lesson plan, 3rd-grade students develop an understanding of the rules of rounding to the nearest 10. The lesson requires one 45-minute class period. The supplies include:

  • Paper
  • Pencil
  • Notecards

The objective of this lesson is for students to understand simple situations in which to round up to the next 10 or down to the previous 10. The key vocabulary words of this lesson are: estimate, rounding and nearest 10.

Common Core Standard Met

This lesson plan satisfies the following Common Core standard in the Number and Operations in Base Ten category and the Use Place Value Understanding and Properties of Operations to Perform Multi-Digit Arithmetic sub-category. 

  • 3.NBT. 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 cents. Should she give the cashier 20 cents or 30 cents?" 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. Ask "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 the right answer. Say "If we changed it to be 20 + 30 + 20 + 80, is that easier?"

Step-by-Step Procedure

  1. Introduce the lesson target to students: "Today, we are introducing the rules of rounding." Define rounding for the students. Discuss why rounding and estimation are important. Later in the year, the class will go into situations that don’t follow these rules, but they are important to learn in the meantime.
  1. Draw a simple hill on the blackboard. Write the numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 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 is used to illustrate the two 10s that the students are choosing between when they are rounding.
  1. Tell students that today the class will focus on two-digit numbers. They have two choices with a problem like Sheila’s. She could have given the cashier two dimes (20 cents) or three dimes (30 cents). What she is doing when she figures out the answer is called rounding—finding the closest 10 to the actual number.
  2. 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 more difficult. That’s where the mental hill comes in.
  3. 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? The answer is 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 backward, which sends you right back to 20.
  4. Continue to do the hill problems with the following numbers. Model for the first three with student input and then continue with guided practice or have students do the last three in pairs: 12, 28, 31, 49, 86 and 73.
  5. What should we do with a number like 35? Discuss this as a class, and refer 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.

    Extra Work

    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:

    • 151
    • 189
    • 234
    • 185
    • 347

    Evaluation

    At the end of the lesson, give each student a card with three rounding problems of your choice. You will want to wait and see how the students are faring with this topic before choosing the complexity of the problems you give them for this assessment. Use the answers on the cards to group the students and provide differentiated instruction during the next rounding class period.