1st Grade Math
Addition and Subtraction
Unit 2
Now that we've gone through place values, we can do harder and more complicated addition and subtraction problems!
Watch this video for an intro into addition and subtraction! -->
Basic Addition and Subtraction (One Digit)
In Kindergarten, when you learned addition and subtraction, you probably used your fingers to count. While this is useful in the beginning, let's try to do these problems in your head. You can visualize your fingers in your head or any object (candy, toys, etc) while doing these problems.
You can also help visualize this with a number line. On the number line above, if you have 6 + 3, start at the 6 and jump forward 3 spaces to get 9. If you have 7 - 4, start at 7 and go backwards 4 spaces to get to 3.
Addition
In addition, you count up all the numbers together to get the total.
Ex. 2 + 5 = 7 Let's say you have 2 action figures. If your parents buy you 5 more action figures, count up how many you have total! The answer is 7 action figures total!
Subtraction
In subtraction, you take away the second number from the first number.
Ex. 8-3 = 5 Let's imagine you have 8 lollipops. If you give 3 lollipops to your friend, how many do you have left? 5! This is what subtraction does. The number of lollipops you have is the first number and the number of lollipops you gave away is the second number. This leaves you with the answer
Let's Try It!
7 + 5 =?
Answer: 12! If we take 7 and add 5, we can count 5 numbers up from 7. This means: 7, 8, 9, 10, 11, 12. 5 numbers were counted above 7, so 12 is our answer.
_?_+ 7 = 14
7; You can get this answer in different ways. You can do 14 - 7 = 7 or you could count how many numbers it takes to reach 14 from 7.
9 - 6 = ?
3; If you take 6 away from 9, you get 3
Play this game to test your knowledge! -->
Two Digit Addition
Now that we understand one digit addition and subtraction, we can go on to a little more complicated and fun problems!
TIP: Structure your work like I have below to make it easier to see which digits have the same place value.
Remember the place values we talked about in the last unit? We can use them for two digit addition and subtraction! When adding, always go from right to left. This means, start with the ones values first and then the tens.
Ex. Let's add together 12 + 14.
Step 1: Start with the ones place (circled) and add those together. In this case, 2 + 4 = 6. Write 6 below the 2 and 4.
Step 2: Go to the tens place (circled) and and add those together. In this case 1 + 1 = 2. Write 2 below the 1s.
Step 3: That's it! Your answer is the number below the line, which is 26!
WAIT! YOU MAY BE WONDERING "BUT WHAT IF THE ONES PLACE DIGITS ADD UP TO A 2 DIGIT NUMBER"??? This is a fantastic question and what we do then is regroup!
Example #2
Here's an example: 14 + 18 = ?
Set up your numbers like this!
Step 1: Again, start at the ones place and add them up. In this case, 4 + 8 add up to 12 (a 2 digit number!). This is where things get weird.
Step 2: Write down the ones digit of the number you get when you add up the ones digit from the problem. In this case, it is 2 (because the ones digit of 12 is 2).
Step 3: Take the tens digit of that same number and write it above the tens place digits of the two numbers you are adding. In this case, it is 1 because that is the tens digit of 12.
Step 4: Add all the numbers in the tens column together (this includes the 1 that you just wrote in). In this case it is 1 + 1 + 1, which equals 3. Write this number down below the tens column.
Step 5: You're done! Your answer is the number below the line. For this example, it is 32!
Two Digit Subtraction
Subtraction is just the opposite of addition, but it has similar steps!
Again, do one digit subtraction from right to left, starting at the ones place!
Example #1
Let's try an example of 15 - 12!
Step 1: Start with the ones place and perform one digit subtraction for those digits. In this case, we subtract 5 - 2, which equals 3. Write this number below the ones place.
Step 2: Go to the tens place digits and do the same thing. In this case, we subtract 1- 1, which equals 0.
Step 3: You're done! The answer is below the line, which is 03, but we can just write that as 3!
Like addition with the regrouping past 10, what happens when the digits subtract to be below 0?? Well, let's do an example!
Example #2
Let's try 16 - 9!
Step 1: Start with the ones place. In this case it would be 6 - 9. This makes a number below 0, so we cannot do that.
Step 2: Instead, we have to split the tens place number into ones. Remember place value? This is where that comes in. 1 ten is equal to 10 ones. Since we have a 1 in the tens place from 16, we can split that ten into 10 ones. This means that we can use that 10 to subtract 9 from. So, we have 10 -9 = 1. However, we still have the 6 in the ones place from 16, so we just add that 6 to the remaining number from 10-9 = 1. In the end, we have 7 in the ones column because 6 + 1 is equal to 7.
Step 3: Since we used the 1 from 16 in the tens column to subtract 9 from, we cannot use it again, so we just cross it out and turn it into 0.
Step 4: You're done! Your answer is 7!
This concept is kind of confusing, so here is an example video!
Addition and Subtraction.mp4