standard_description: Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
problem_activity: Narrative
The purpose of this activity is for students to learn stage 2 of the Check It Off center. Students take turns picking two number cards that are multiples of 10 (0–90) and choose whether to make an addition or subtraction expression. Students check off the value of the sum or difference (0–90) and then write the addition or subtraction expression on the recording sheet.
Required Materials
Materials to Gather
Connecting cubes in towers of 10 and singles
Materials to Copy
Number Cards, Multiples of 10 (0-90)
Check It Off Stage 3 Recording Sheet
Required Preparation
Create a set of cards from the blackline master for each group of 2.
Launch
Groups of 2
Give each group a set of cards, two recording sheets, and access to connecting cubes in towers of 10.
“We are going to learn a new way to do the Check It Off center. It is called Check It Off, Add or Subtract Tens.”
“Choose two cards and decide whether you want to add or subtract. After you find the sum or difference, check it off on the recording sheet and write the expression. Take turns with your partner. At the end of the game the partner who checks off the most numbers wins.”
Activity
10 minutes: partner work time
Activity Synthesis
Display a recording sheet with 90 checked off and 10 not checked off.
Display cards 40 and 50.
“Should we add or subtract these numbers? Why?” (We should subtract because
\(50 - 40 =10\)
and we haven’t checked off 10 yet, but
\(40 + 50 = 90\)
and 90 is already checked off.)


aligns: no