How to Add 2-Digit Numbers
Hi, I’m Miss Erica and today’s lesson is on adding two digit numbers. First let’s do a quick review of adding 1 digit numbers. Here we have 5 plus 3, illustrated by 5 dots plus 3 dots. When we add them together, we get 8. Here’s one more, 9 plus 8 again, we have 9 dots plus 8 dots and that gives us 17 dots; so our sum is 17.
Now when you start to add two digit numbers, it becomes a bit more difficult to add by drawing dots; for example, if you do this 34 plus 25, you have to draw 59 dots to get your answer. That takes a lot of time and energy. So let’s look for an easier way to do this kind of problem. One way to make the problem easier is to pick a symbol to show a group of dots. For this lesson, I will use a box like this to show a group of 10. Since the box is the same as a group of 10 dots, we will call it a 10 box. So every time we have a group of 10 dots, we will replace it with a 10 box.
So if we want to show the number 40, we can draw 40 dots or we can use the short cut and simply draw four 10 boxes. Now, count by 10’s and we have 10, 20, 30, and 40. Let’s practice that a couple more times. To show the number 17, we could draw 17 separate dots or we can replace our group of 10 with the 10 box; and we have 7 leftover, since there are only dots in that group, we can’t replace it with a 10 box. So we have one 10 box and 7 dots; a total of 17. Let’s try that with the number 63. We could draw 63 separate dots or we can replace each group of 10 with a 10 box. 10, 20, 30, 40, 50 60 and we have 3 leftover, so we have six 10’s and 3 dots; a total of 63.
Now let’s go back to our equation, we want to find the sum of 34 and 25. In the number 34, we have three tens; so we draw three 10 boxes. And we have four ones, so we draw 4 dots. Now in the number 25, we have two tens, so we draw two 10 boxes and we have 5 ones, so we draw 5 dots. When we add 2 digit numbers, we always add our ones first. So let’s count up the total number of dots; 1, 2, 3, 4, 5, 6, 7, 8, 9, we have 9 dots, so we write a 9. And now we count up our number of tens. 1, 2, 3, 4, 5; we have 5 tens, so we write a 5, and that give us our sum 59.
Now, let’s look at another way to solve this same problem, again we want to add 34 and 25, so let’s create a chart with 2 columns. The first column shows tens and the second column shows ones. In the number 34, we have three tens so we write a 3 in the tens column; and in the number 34, we have four ones so we write a four in the ones column. Now, in the number 25, we have 2 tens, so we write a 2 in our tens column and we have 5 ones, so we write a 5 in our ones column. Remember, we should always add our ones first; if you have trouble remembering that, try using this rhyme, first you add the ones, they’re on the right you know; then you add the tens and on and on you go. So let’s add the numbers in the ones column, 4 + 5 = 9; so we write 9.
Now we add the numbers in our tents column, 3 + 2 is 5; so we write a 5; and that gives us our sum 59. Let’s try another problem and use both ways to solve it. Here we are doing 43 + 31. Let’s start with the illustration method that uses boxes and dots. In the number 43, we have four tens, so we draw 4 ten boxes and we have 3 ones so we draw 3 dots. Now in the number 31, we have 3 tens, so we draw 3 ten boxes and we only have 1 one, so we draw 1 dot. Now let’s count up the total number of dots; 1, 2, 3, 4 so we write a 4, and now we count up a number of ten boxes; 1, 2, 3, 4, 5, 6, 7; we write a 7. Do you see our sum? 74.
Now let’s try the same problem using the chart method, we create our chart and we add the numbers. In the number 43, we have 4 tens and we have 3 ones; and in the number 31, we have 3 tens and we have 1 one. Now remember, first you add the ones, they’re on the right you know, then you add the tens and on and on you go. So we add our ones column, 3 + 1 = 4; you write a 4 and then we add our tens; 4 + 3 is 7; so we write 7. And that gives us our sum, 74. Here are some problems you can practice on your own, if you’re having trouble with the chart method, you can try illustrating the problems with boxes and dots. But as you move on to a more difficult problem, the chart method will be the quickest and easiest for you to use. So just keep practicing until it feels easy.
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