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This Number Talk encourages students to look for and make use of the structure of numbers in base-ten to mentally solve division problems (MP7). The reasoning elicited here will be helpful later in the lesson when students divide large numbers using increasingly more abstract strategies. In explaining their reasoning, students practice being precise in their use of language (MP6).
Find the value of each expression mentally.
The purpose of this activity is for students to encounter a way to divide a multi-digit number by using partial quotients and writing equations for them. They analyze and interpret the equations and consider how it is like and unlike finding quotients using base-ten representations. In the next activity, students will be introduced to a way to record partial quotients vertically.
Here is how Priya finds the value of .
Describe the steps in Priya’s method.
The purpose of this activity is to introduce students to an algorithm that uses partial quotients, a vertical method of recording partial quotients. Students compare and contrast this approach with other ways of dividing numbers using partial quotients. They also use partial quotients to divide multi-digit numbers.
When students analyze Priya and Tyler's work and explain their reasoning, they critique the reasoning of others (MP3).
Tyler uses a different method to find the value of . Let’s compare Priya’s and Tyler’s work.
Priya's method
Tyler's method
Why do you think Tyler uses subtraction in his method?
“Today we learned to use an algorithm that uses partial quotients to divide numbers.”
“How would you explain ‘partial quotients’ to a classmate who might be absent today?” (We can find a quotient in parts—dividing a portion of the dividend at a time—until there is no more or until there is not enough of the dividend to divide. Each quotient is called a partial quotient.)
“Suppose we’d like to find the value of and know we could decompose the 738 into parts. How would we know what numbers to choose?” (Look for multiples of 9. Try to start with the largest multiple of 9 and 10 within 738.)
“What are some ways to decompose 738 into multiples of 9?” (, or , among others.)
Display:
“We saw two ways of recording partial quotients—by writing a series of equations and by recording the steps of division vertically. Where can we see the partial quotients in each one?”