Warm-up: Number Talk: Same/Different (10 minutes)
The purpose of this Number Talk is to elicit strategies and understandings students have to divide whole numbers by decimals. These understandings help students develop fluency and will be helpful later in this lesson when students divide decimals greater than 1 by decimals less than 1.
- Display one problem.
- “Give me a signal when you have an answer and can explain how you got it.”
- 1 minute: quiet think time
- Record answers and strategy.
- Keep problems and work displayed.
- Repeat with each problem.
Find the value of each expression mentally.
- \(20 \div 2\)
- \(2 \div 0.2\)
- \(50 \div 2\)
- \(5 \div 0.2\)
- “Why do \(50 \div 2\) and \(5 \div 0.2\) have the same value?” (Because 5 is 50 tenths and I am dividing that into groups of 2 tenths so that's \(50 \div 2\).)
Activity 1: Dividing by a Tenth and a Hundredth (15 minutes)
The purpose of this activity is for students to divide decimal numbers by 0.1 and 0.01. They are given diagrams to help see that there are 10 tenths in each whole and 100 hundredths in each whole. The diagrams are not labeled with the whole so that the same diagram which shows \(1.6 \div 0.1 = 16\) can be interpreted as whole number division showing \(160 \div 10 = 16\). This dual way of interpreting one diagram is highlighted in the synthesis. When students interpret the diagram as representing two different equations they attend to precision in the meaning each part of the diagram (MP6).
Advances: Representing, Conversing
- Groups of 2
- 5 minutes: independent work time
- 5 minutes: partner work time
- Monitor for students who:
- Describe how Jada's diagram shows the value of \(1.6 \div 0.1\) as 16.
- Describe how Jada's diagram also represents the expression \(160 \div 10\).
- To find the value of \(1.6 \div 0.1\), Jada drew this diagram.
Describe how the diagram shows 1.6.
- Describe how the diagram shows 16 groups of 1 tenth.
- Describe how the diagram shows the value of \(1.6 \div 0.1\).
- Describe how the diagram also represents the expression \(160 \div 10\).
- Explain how this diagram represents \(1.3 \div 0.01\).
- What is the value of \(1.3 \div 0.01\)? Explain or show your reasoning.
- Ask selected students to share their reasoning for each problem.
- Display: \(1.6 \div 0.1 = 160 \div 10\)
- “How does the first diagram show that this equation is true?” (If each large square is a whole then the number of shaded strips is \(1.6 \div 0.1\) and if each large square is 100 then the number of those strips is \(160 \div 10\). The same diagram represents both expressions so they are equal.)
- Display: \(1.3 \div 0.01 =130 \div 1\)
- "How does the second diagram show that this equation is true?" (If each large square is a whole then the number of small pieces represents \(1.3 \div 0.01\) and if each large square is 100 then the number of small pieces represents \(130 \div 1\).)
Activity 2: Divide Decimals by Decimals (20 minutes)
In this activity, students practice finding quotients of decimals divided by 0.1 and 0.01. Students find the value of different expressions without the scaffold of a diagram. Monitor for these approaches:
- whole number quotient facts
- multiples of the divisor
Supports accessibility for: Conceptual Processing, Attention, Organization
Materials to Copy
- Small Grids
- Groups of 2
- Give students access to blackline master of grids.
- 8 minutes: independent work time
- 2 minutes: partner discussion
- \(5 \div 0.1\)
- \(5 \div 0.01\)
- \(0.5 \div 0.1\)
- \(0.5 \div 0.01\)
- \(0.02 \div 0.01\)
- \(1.53 \div 0.01\)
\(0.5 \div 0.1 = 5\)
\(0.50 \div 0.01 = 50\)
- “How can we use the meaning of decimal place values to explain these equations?” (5 tenths is the same as 50 hundredths so that’s 5 groups of 0.1 or 50 groups of 0.01.)
- “How can we use the meaning of decimal place values to help find the value of \(1.53 \div 0.01\)?” (The three is in the hundredths place so there are 3 one hundredths in three hundredths. The 5 is in the tenths place and there are 10 hundredths in each tenth so that's 50 more hundredths. There are 100 hundredths in one whole. That's 153 hundredths altogether in 1.53.)
“Today we divided a decimal by a decimal and then found lots of quotients involving decimals.”
\(1.25 \div 0.01 = 125\)
“How do we know this equation is true?” (If we multiply the dividend and the divisor by 100, we get \(1.25 \div 0.01 = 125 \div 1\), which is 125. We can also see that there are 100 hundredths in 1, and 25 hundredths in 0.25, so there are 125 hundredths in 1.25.)
“How is dividing with decimals the same as dividing with whole numbers? How is it different?” (I can use multiplication in both cases. I can draw a diagram in both cases. I use place value in both cases. With decimals I need to think carefully about the meaning of each digit. I think the diagrams are more helpful to get started with decimals to visualize the numbers I am working with.)
Cool-down: Divide by Decimals (5 minutes)
Student Section Summary
In this section we learned to divide with decimals. We studied different ways to find a quotient like \(3 \div 0.1\). We can draw a diagram which shows that there are 10 groups of 0.1 in each whole so there are \(3 \times 10 \) or 30 groups of 0.1 in 3 wholes: \(3 \div 0.1 = 30\).
We can also think about place value. We know 3 is 30 tenths and 0.1 is 1 tenth, so \(3 \div 0.1\) is equivalent to \(30 \div 1\) which has the value 30. We also can use multiplication to find the value of \(3 \div 0.1\). We know that \(10 \times 0.1 = 1\) and \(30 \times 0.1 = 3\) so this also shows that \(3 \div 0.1 = 30\).