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Math for Kids23 chapters | 325 lessons

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Instructor:
*Audrey Akins*

Audrey has more than a decade of experience teaching elementary. She has a bachelor's in journalism and a master's in education.

When you began to add and subtract, you may have learned using objects you can hold, like blocks. In this lesson, you'll learn what an algorithm is you can move from using objects to using an algorithm.

An **algorithm** is a step-by-step way of solving math problems faster than counting one-by-one. It's a set of instructions you use to find the answer faster, much like following directions to the store or following the instructions in a recipe for cookies. In this lesson, we'll look at the algorithms for addition, subtraction and multiplication.

The standard algorithm for addition has two basic steps:

- Arrange the numbers vertically (on top of each other) so the place values line up (the ones places are in the same column, the tens places are in the same column, and so forth).
- Starting from the right, add the digits in each place value. Carry over sums larger than 10 as you go, if necessary.

Say you have 17 oatmeal cookies and 34 cupcakes. You want to know how many total snacks you have. Let's work through our algorithm:

1. Arrange the numbers vertically so that the place values line up:

2. Starting from the right, add the digits in each place value. Carry over sums that are 10 or larger as you go, if necessary:

Notice that, since 7 + 4 is 11 (greater than 10), we carried over the 1 into the tens place. Then, we added the tens places (including our carried over number) to find out answer: 51.

The standard algorithm for subtraction is similar to the algorithm for addition. Let's look at each step:

- Arrange the numbers vertically so the place values line up. Make sure the number you are subtracting from is on top, and the number being subtracted is on the bottom.
- Starting from the right, subtract the top digit from the bottom in each place value. Borrow from the larger place value when necessary.

Your best friend gives you 56 Skittles and tells you that 42 of them are green and the rest are orange. To find the number of orange skittles, you can solve for 56 - 42. Let's use our algorithm:

1. Arrange the numbers vertically, with the number being subtracted from on top:

2. Starting from the right, subtract the top digit from the bottom in each place value. Borrow from the larger place value when necessary:

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You'll notice that, in this example, we didn't need to borrow from the larger place value. If the top digit were smaller than the bottom digit, you would need to borrow from the top digit in the left column.

Things get a bit more complicated with the multiplication algorithm, and the steps can change depending on how many digits are being multiplied. Let's look the steps for multiplying a two-digit number by a one-digit number:

- Arrange the numbers vertically so that the place values line up.
- Multiply the one-digit number by the ones place of the two-digit number. Write the answer below the line.
- Multiply the one-digit number by the tens place of the two-digit number, then add a zero to the end of that product. Write this answer below the line as well, making sure the place values line up.
- Add the two products below the line to find your answer.

Say that you mowed 6 lawns over the summer to earn money for a new bike. You charged your customers $12 for each lawn. How much money did you make over the summer? Let's work through the steps of our algorithm:

1. Arrange the numbers vertically so that the place values line up:

2. Multiply the one-digit number by the ones place of the two-digit number. Write the answer below the line:

3. Now, multiply the one-digit number by the tens place of the two-digit number, then add a zero to the end of that number. Write this answer below the line as well, making sure the place values of the answers line up:

4. Lastly, add use the addition algorithm to add the two answers together:

An **algorithm** is a step-by-step approach to solving problems. We can use algorithms to solve addition, subtraction, and multiplication problems, among others.

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Math for Kids23 chapters | 325 lessons

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