Lesson 12 Analyze Subtraction Algorithms

- Let’s think about subtraction algorithms in more detail.

Warm-up Number Talk: Subtract within 1,000

Find the value of each expression mentally.

$400 - 200$
$450 - 200$
$450 - 205$
$450 - 215$

Activity 1 Compare Two Subtraction Algorithms

The first steps of two algorithms are shown.

Algorithm A, step 1

Subtraction. Five-hundred and eight minus one-hundred fifty-six.

Algorithm B, step 1

Subtraction. Five-hundred and eight minus one-hundred fifty-six equals two.

How are the steps different?
Use each algorithm to find the value of $824 - 541$ .

Activity 2 Use an Algorithm?

Noah wanted to find the value of $301 - 167$ and wrote:

Subtraction. Three-hundred and one minus one-hundred sixty-seven.

Elena said that we can’t subtract this way because we would need more ones to subtract 7 ones, but there’s a zero in the tens place of 301.

Do you agree with Elena’s statement? Explain your reasoning.
Show how you would use an algorithm (either Noah’s or another algorithm) to find the difference between 301 and 167.

Practice Problem

Problem 1

Use an algorithm of your choice to find the value of each difference.

$218 - 136$
$473 - 258$