We warm up with several partners of 100 addition and subtraction problems to solve mentally (2NBT.B8).
I ask, "How much is 40 + 60?" They record the answer in their math journal.
I follow with 100 – 70, 80 + ? = 100, 100 – 10 = , and 50 + ? = 100.
With each problem students share their solutions to the problems, telling not only the answer but also the way they found the answer.
Next, I ask them some plus and minus 10 problems within 100.
I ask, "How much would 85 + 10 be? What would 76 – 10 equal?" I finish with 63 +10 = ? and 92 – 10 = ?.
I ask the students, "How did you know what to do with the numbers in each problem?" (The +/- signs.)
I tell them that the +/- are like road signs. A stop sign tells us to stop our car. The +/- signs tell us what to do with the numbers in a math problem.
I put the following on the board: 60 + 40 = 100
Underneath, I write: 60 – 40 = 20.
Why did 2 problems with the same numbers have different answers? The +/- signs tell us what to do.
Today we are going to see what to do when the road signs aren’t so clear.
I invite students to come to the rug area. On the white board (easel, overhead or poster) I put the + symbol over one column and the - symbol over the other column. I tell students that today we will look for words that help us to figure out which road sign to use. I want students to attend to the structure of a word problem and find the words that would best help them to solve the problem. (MP7)
I post this problem at the bottom of the white board.
I read 63 pages of my book on Monday. On Tuesday I read 10 more. How many pages have I read altogether?
I ask students if there are any words in the problems that could act as street signs?
Responses can include how many, altogether as addition road signs). I ask if anyone can come and write the number sentence for the problem on the board? (63 + 10 = 73). I put the words how many, altogether in the + column.
Next, I post this problem:
I started with 48 cards. I gave 20 to a friend. How many are left?
Again, I ask which words might be the street sign? (How many left?) I ask if anyone can come up and write the number sentence for this problem? (48 – 20 = 28). I put the words how many left in the – column.
I post one more problem on the board:
I build a tower with 35 blocks. My friend builds a tower with 25 blocks. How many more do I have?
I ask if anyone can come up and write the number sentence? (35 – 25 = 10 or, they may write 25 + ? = 35.) If they write the first choice, I put the words how many more under subtraction. If they give the second choice, I ask how might we solve the problem? We can count up from 25 to 35, or count down from 35 to 25. This is a comparison problem. We don’t want to add the two numbers, so we put the words how many more in the subtraction column.
I tell students that today they will work with a partner to solve some word problems. worksheet - where is question.docx They will look for the words that help them know what to do (the street signs) and record these words at the top of their papers so that we can put them all together to make a class chart of math street sign words.
I partner students heterogeneously so that they can help one another find the math street sign words.
I give students directions and then circulate around the room to support students who may be struggling.Finding the Words
Common Core Standard 2NBT. B 5 says “Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.” My goal is for students to increase their understanding of the properties of operations and the relationship between addition and subtraction as they solve problems today.
I also expect them to be able to explain why the addition and subtraction strategies they have chosen work to solve the problem (2NBT. B 9). Explaining The Solution
I repost the chart I started with at the beginning of the lesson and ask for any other words that students may have found in the word problems that helped them know which street sign (+-) that they should use to solve a given problem. We post the chart in the classroom for students to refer to later.