I write two problems, each written vertically, on the board:
82 - 56 = 34
82 - 56 = 26
I ask students to take out their math journals and check the 2 problems. I tell them to decide which one is correct and which one is not and why. This is asking students to construct a viable argument and critique the ideas of others (MP3).
I am looking to see if students will just flip the 2 and 6 in the first problem to solve 6 - 2 = 4 in the ones place and 80 - 50 = 30 for the tens place.
I give students a few minutes to solve the problems and then I ask them to close their eyes and vote on which one is correct. After we vote I ask for volunteers to explain why they thought one or the other were correct.
We discuss the ways students figured out the problem.
Today I hand each student a small stop sign with eyes and a check mark on it.stop look check.docx I tell them that when I subtract I first need to go to the ones column (written vertically) and stop (I draw a stop sign around the number of ones to be subtracted), use my eyes to look, and check (I draw a check mark on the bigger number of ones.) If the check mark is on the top, I can subtract because the bigger number is on top. If the check mark is on the bottom, I have to borrow because the bigger number is on the bottom.
I hand each student a paper with 4 double-digit subtraction problems on it. I ask them to stop, and draw the stop sign around each lower number, look at both digits in the ones place and check the biggest one. I am trying to teach students to attend to the structure of the problem (MP 7) as they learn more about regrouping.
Now if the check mark is on the top they can subtract, but if it is on the bottom, they will need to borrow a ten first. I point out that bottom and borrow both begin with B to help us remember which is which.
After I see that everyone has stopped, looked and checked all 4 problems, I ask them which of the problems they will need to borrow on. Then I remind them of the base ten or tens frame drawings that they can use to solve the problems.
I give them time to solve the problems and then we share what we did.
I write a series of double-digit subtraction problems on the board. I ask students to copy them onto a blank piece of paper and then use the stop, look and check process to decide whether they need to borrow or not.
I tell students they can then use their favorite strategies to complete the problems.
I give students time to complete the problems on their own. I circulate around the room to listen to student thinking and to help struggling students.
I ask students to pair up with the person across from them and to share their solutions to the problems. I ask them to look and see if they needed to borrow, and then to share answers. If they did not agree, they should talk about how the problem could be done, and then use another method to find an answer together.