Lesson 2

This Number Talk encourages students to think about sums that make 10, 100, and one whole and look for ways to use these sums to mentally find the value of different expressions with whole numbers and fractions. The understandings elicited here will be helpful throughout this unit as students add and subtract whole numbers fluently and add and subtract fractions with the same denominator.

When students identify ways to make 1, 10, or 100, they look for and make use of the properties of operations and the structure of whole numbers and fractions (MP7).

• Display one expression.
• “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 expressions and work displayed.
• Repeat with each expression.

• “How were these expressions the same? How were they different?” (They each have \(38 + 62\), so you can look for ways to make 100 in each. The last three expressions all have a mixed number with sixths. Some expressions did not have fractions. One expression had fractions that had a sum that was equivalent to 1 whole.)
• As needed, “How could we look for ways to make 100 in each expression? How could we look for ways to make 1?”

The purpose of this activity is to represent and solve a measurement problem with fractions. Students may approach this activity in multiple ways and are invited to apply what they know about operations with fractions, comparing fractions, and fraction equivalence to make sense of and solve the problems (MP2). Throughout the activity, listen for the ways students use what they know about comparing fractions and fraction equivalence as they make sense of the problem. Although students may consider ways to subtract fractions with unlike denominators, this is not a requirement for grade 4. Focus the conversation during the activity and the synthesis on how students can solve the problem by reasoning about equivalent fractions and the representation they use to make sense of the problem.

• Groups of 2

• 5 minutes: independent work time
• 5 minutes: partner work
• Monitor for different representations of the situation (for example, equations, drawings, and number lines).

If students create representations that do not match the quantities in the problem, consider asking:

• “How does your representation show the length of the straw? How does it show Noah and Tyler’s pieces?”
• “How does your representation show the action in the situation?”

• Invite previously selected students to share their arguments and allow peers to extend and add ideas to the conversation.
• “How does each representation match this situation about the straw?” (They each show the total length of the straw and ways it could be broken into smaller parts. They each show the length of Noah and Tyler’s pieces. They show how long Jada’s piece could be.)

In this activity, students practice solving word problems that involve adding and subtracting mixed numbers. Students interpret fractions in the context of comparing heights and use what they know about decomposing whole numbers and equivalent fractions to make sense of and solve each problem (MP2). Look for the different ways students use what they know about the structure of whole numbers and fractions as they reason about how to solve each problem and share their thinking with others.

• Groups of 2
• “Have you ever ridden a ride at an amusement park or a fair? Have you ever not been able to ride a ride because you were too young or not tall enough?”
• As needed, explain that some rides require the riders to be a certain height for their safety.
• “Let’s solve some problems about height and amusement park rides.”

• 10 minutes: independent work time
• 5 minutes: partner discussion
• “Compare your strategy with your partner’s strategy.”
• Monitor for students who use the relationship between addition and subtraction to represent the situations.

If students add or subtract quantities in each problem in ways that do not match the situation, consider asking:

• “Who should be taller _____ or _____? How do you know?”
• “Does your answer match the heights in the situation? How do you know?”
• “How could you use a diagram to help you represent the different heights in this problem?”

• Invite students to share their equations and explain their solution for Lin’s height.
• Display: \(50\frac{5}{8} + \frac{18}{8}\)

• “How might you reason about how to find the value of Lin’s height?” (Think about adding \(\frac{3}{8}\) to \(\frac{5}{8}\) to make 51, then it'd be \(51\frac{15}{8}\). We are adding more than 1 because \(\frac{18}{8}\) is more than \(\frac{8}{8}\).\(\frac{18}{8} = \frac{8}{8} + \frac{8}{8} + \frac{2}{8}\) or \(2\frac{2}{8}\), and \(50\frac{5}{8} + 2\frac{2}{8} = 52\frac{7}{8}\))

  (\(50\frac{5}{8} + \frac{18}{8} = 50\frac{5}{8} + 2\frac{2}{8} = 52\frac{7}{8}\))

• Select students to share both addition and subtraction equations for Mai’s current height and discuss how both equations (\(54 - 51\frac{7}{8} = 2\frac{1}{8}\) and \(54 = 51\frac{7}{8} + 2\frac{1}{8}\)) could be used to represent this situation.