Addition with decimal numbers with 1 or 2 decimal places

General

Students learn to add decimal numbers with 1 or 2 decimal places. They also learn to determine which numbers to use from a story problem.

Common core standard(s)

5.NBT.B.7

Relevance

Discuss with students that it is important to be able to add decimal numbers with 1 or 2 decimal places because you can then calculate how much something costs for example.

Introduction

Students practice by setting decimal numbers on the number line. Do this by drawing a line from the decimal number to where it belongs on the number line.

Development

Using the number line, explain how you add decimal numbers with one decimal place together. Show that you leave the first addend (56.2) whole and you split the second addend (31.7) into two parts, 31 and 0.7. Using the number line, add 31 to the first addend. Then add 0.7 on the number line. Next show an alternative method of adding the decimal numbers together. You split the first and second addend into the number before the decimal point, and the number after the decimal point. That means that, with 56.2 + 31.7 you get the addition problems 56+31 and 0.2+0.7. Calculate the answer to both of these problem and then add the two totals together to find your total. Use the next exercise to check that students understand adding decimal numbers with one decimal place. Next tell students how to add decimal numbers with two decimal places. Show the addition problem 42.83 + 27.55. Explain how to solve this problem using both methods used for solving with one decimal place. Check that students understand how to add with two decimal places by doing a few problems together. Make sure to tell students to use the method they like best when solving for the totals.
Next comes an exercise in story form. Using the problem with cola bottles, show which steps they should take to solve a story problem. Have them first determine what kind of math problem it is, then determine which numbers are important to the math problem, then to say or write the problem, and finally to solve the problem. Give another story problem and ask students to solve it using the steps given.

Check that students understand adding decimal numbers with 1 or 2 decimal places by asking the following questions:
- Which methods can you use to solve an addition problem with one or two decimal places?
- Which method do you like best? Why?
- How do you solve a story problem?
- Which steps do you take to solve a story problem?

Guided practice

Students start with an addition problem with a decimal number with one decimal place, then they are given an addition problem with two decimal places, and then they are given a story problem to solve.

Closing

Check that students are able to say which method they use to solve adding a decimal number with 1 or 2 decimal places. Also have students explain which steps to take to solve a story problem. Check that students have understood by doing a few problems together. Emphasize that students should use the method they feel most comfortable with. Finally they see a number line with a figure at zero. Have the class come up with a number for the figure to jump to. Repeat this again. Then add those two jumps together.
Example: You first select the frog and jump 0.4. You next select the duck and jump 24.5. As a class you solve 0.4+24.5= 24.9

Teaching tips

Students who have difficulty adding decimal numbers with 1 or 2 decimals can practice with putting decimal numbers on the number line. Give the students a decimal number with 1 decimal point and then ask them to add 0.1 each time and draw that on the number line. Also be sure to practice the jump from 0.9 to 1. Once they are comfortable setting decimal numbers with 1 decimal place they can move to decimal numbers with 2 decimal places.