My class has been working hard on using mental math to add ten and subtract ten. I will review this concept and get their minds on math by using base ten blocks:
(While holding up 3 base ten blocks and 4 ones units.)
Students, what number do I have here? (34)
I want to add ten, so I am going to put another base-ten block with it, what do I have now? (44)
(If necessary, start with the 34 and point and count on the base ten block to add ten more)
I am going to put one base-ten block down, what do I have now? (you have 34 again)
What if I subtract ten from 34 and take one base ten away? (You have 24)
(If necessary, start at 34 and count backwards while pointing at the sections on the base ten block.)
If it is difficult for your students to see the tiny sections of the base-ten block, use unifix cubes instead. You could even snap together different colored cubes, so they can see each unit easily.
I pick two more numbers and follow the same progression.
Students must develop a strong understanding of place value to use tens and ones to assist in addition and subtraction of 2-digit numbers. The Common Core Standards want first graders to be able to use their knowledge of place value as a strategy for completing addition and subtraction with 2-digit numbers. This requires being able to solve 2-digit addition problems by adding the ones to the ones and the tens to the tens. Today's lesson continues to help students understand how to figure out 10 more or 10 less without having to count (1.NBT.C.5). To do this, I want to help them identify the pattern that occurs on a 120 chart and show them how each column increases or decreases by ten as they move up and down. I want them to identify this pattern because it can help them internalize the connection between the value and the digit in the tens place as it changes by one (MP7). I am selecting the 120 chart as a tool for them to solve their problems, but my goal is for them to see this tool provides a strategic method to find their answer.
To begin, I will ask a student to stand with a pointer and point out the numbers as the class helps me count down different columns.
The patterns I want them to identify are:
I will have them practice by playing a game of I have, Who Has. To play this, cut the cards apart and pass them out. Have the 120 chart pulled up on the SmartBoard or have one posted in your class or on their desk for them to refer to. Find the student that has the beginning strip and start to play the game.
My concrete learners have moved beyond using base-ten blocks to solve these problems and are relying on the more abstract too, the 120 chart, for support. I will have some students who need additional teacher support in using the chart correctly through the game for them to be successful. I will be walking around the room and assisting those that need it with their chart. It is very important to know your students' abilities and be ready to guide them towards an answer. For instance, some get lost in multiple step problems. I will assist by pointing out numbers on the 120 chart and helping them identify whether to move up or down to find their answer. I will do this by reminding them what it says on their play card: 10 more or 10 less.
There are several different online sources to use for your students to practice this skill. No matter which sheet or online activity you select, make sure you point out to your students they need to pay attention to whether they are subtracting or adding. Because these problems are complex for first grader (they involve the application of both place value concepts and operations), students might miss the plus or minus signs. Here is an example of a student who used addition instead of subtraction. This is one student I will need to check in with next time to make sure the student sees the error.
Below are several different practice pieces to access online:
I have dry erase boards in my room that we can use for quick practice work. If you do not have these, you can put a sheet of paper inside a page protector and still use dry erase markers. It comes off just as easily.
To close out our lesson I will pass out our dry erase boards and markers and provide a few problems for my students to solve involving adding or taking away 10 from a 2-digit number. I want them to write their answer and hold their board in the air for me to see. This way, I can quickly assess how my student have done with mastering today's material and plan future instruction.