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## K.OA.2

This is Common Core State Standards Support Video for Mathematics. The standard is K.OA.2.

The standard reads, solve addition and subtraction word problems, and add and subtract within 10, e.g., by using objects or drawings to represent the problem. Now the best use of time and effort for this standard would be to use the contextual problems that are involved as part of your regular instruction, especially in conjunction with the counting and cardinality domain, and the operations and algebraic thinking domain. For example, the third standard in the counting and cardinality domain is focused on writing numbers from 0 to 20. In standard 7 of that same domain, students compare two numbers between 1 and 10 that are presented as written numerals. In the operations and algebraic thinking domain, the third standard states the expectation is that students will decompose numbers less than or equal to 10 into different pairs and that will be an integral part of how they solve the word problems. For operations in algebraic thinking, the introductory statement reads: understand addition as putting together and adding to, and understand subtraction as taking apart and taking from. So that's how we'll approach these word problems.

Let's try this problem. Last week, Angela received five stars for good behavior, this week she received four stars for good behavior. How many has she received all together? So you would want some type of manipulatives, you know some kind of a little stars, cutouts or whatever that you know that you can come up with. So students will start with one, two, three from last week and then four, five, six. The last one to count would be seven, so they know all together that Angela got seven stars for good behavior. Now notice here's another connection to a standard in this same grade level. Standard K.CC.4 states: understand the relationship between numbers and quantities; connect counting to cardinality. In particular standard b says, understand that the last number name said, tells the number of objects counted. The number of objects is the same regardless of their arrangement or the order in which they were counted. So again, notice the connection that when students solve this problem that way by counting, you're also addressing this standard. So again a contextual based standard like this one, where we're talking about solving addition and subtraction word problems, something in context, you're able to address other standards simultaneously.

Now going back to the third standard K.CC.3, that one was focused on writing numbers from 0 to 20, so you might also have students do this so when we set up the problem we can write down the symbols. You know 3 for the three stars here and 4 for the four stars over here and of course with the + symbol to indicate addition then the = sign to indicate you know the solution. On the other side, if we take 3 + 4, and we add them all together obviously there's 3 and then there are 4 more when we connect all of these together and count we have a total of 7. So have students write the symbolic representation for seven for the solution.

Let's try a different problem. Blake's mother bought six apples one afternoon. Blake ate one and his sister ate one also. How many apples are left? Now notice the way this problem is set up. We have Blake eating one and his sister ate one also. The reason that this problem is deliberately set up this way, is that in the future students are going to hit a little bit of an obstacle because they get used to having problems where it's just one-step. But when they get to multistep problems, that involves two or more steps, and then it gets a lot more difficult. Doing something like this will help to get some experience with that. So rather than saying for example that Blake ate two apples, we set it up this way to again actually make it a multistep problem. Now the expectation here would be okay you start off with six apples and Blake ate one and his sister ate one. So we're left with one, two, three, four, and that's pretty much you know the standard way of solving it. What you would also want students to do is to write the symbolic representations to where you would have something like this. But we have a problem, because that’s just too over the top, and that is too much for kindergarten students to handle. So really what you would want to do is treat this like two separate problems. So let's start with Blake. So Blake eats one apple. So this takes care of Blake. So what happens now is that we have this many left, which is of course five apples. Have students write down the symbolic representation. Now we can go on to the second part of the problem where we now have five apples and his sister eats one. So we have this situation now, and now students can finish out the problem. She ate her apple so we're left with this many, which of course would be four apples that are left.

Now this is really easy to do, don't overlook sums and differences involving zero. I know it sounds a little bit simplistic, but students need practice with this. So let's look at this problem. There are seven red blocks on the table. The teacher put them all away in the closet. How many red blocks are left on the table? Well symbolically what would happen here is that all of the red blocks were taken away from the table and put away. So they're gone and our solution is zero. Again this is important because you're laying the foundation for the idea of the additive inverse; when you subtract something from itself you'll always get zero. Let's look at another example.

There are five blue blocks on the table. The teacher forgot to put them away in the closet. How many blue blocks are left on the table? Well none got taken away, so we still have the same amount of blocks left on the table. So it's actually five subtract zero, for a solution of five. So again you're laying the foundation for the idea that when you add or subtract zero, you still get the same solution.