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Amy has a master's degree in secondary education and has taught math at a public charter high school.

After watching this video lesson, you will get better at adding and subtracting numbers in your head without a calculator. Learn how breaking apart your addition and subtraction problems makes them easier and quicker to solve in your head.

Mental Math

Mental math is about performing math calculations in your head without a calculator or paper. This is a good skill to have because in some math tests, you have to work out the problems without the use of a calculator. Being able to solve addition and subtraction problems without a calculator will help you to pass these tests successfully. And also, in real life, you will be able to work your way through many situations on the go. For example, you will be able to quickly figure out your total cost when you are shopping or when you are eating out. Say you wanted to buy two items at the store. One costs $15 and the other costs $35. You can use mental math to help you quickly add these two costs up to find your total. Let's take a look.

Addition

Mental math involves breaking up your problem into parts that are easy to handle. So, when you need to add $15 and $35, you can break each number up so that each number becomes an addition problem on its own. The $15 can be broken up into $10 and $5. The $35 can be broken up into $30 and $5. Now, you can go ahead and add the tens and then you can add the single digits. Adding up tens is a lot easier than adding the two numbers just like that. $10 + $30 is an easy problem to do. You can look at it right away and see that it equals $10 + $30 = $40. What about $5 + $5? Can you do that problem quickly in your head without a calculator? That's right. It equals $5 + $5 = $10. Now, to find your answer, you add up the parts. $10 + $30 = $40, and $5 + $5 = $10, so your answer is $40 + $10 = $50. This means that $15 + $35 = $50. And you are done! The total cost to purchase your two items that cost $15 and $35 is $50.

Subtraction

Let's look at another problem. This time, we will see how mental math works with subtraction.

You have just counted your piggy bank money and you have a total of $92. You have been saving up so you can buy this game that costs $43. After buying this game, how much will you have left in your piggy bank?

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To solve this problem, you need to subtract your $43 from your $92. Using mental math to solve this subtraction problem is a little bit different than when you are solving an addition problem. When working with your subtraction problem, you break up the number that you are subtracting into its parts and then you subtract each part starting with the smallest. Breaking up the $43, you get $40 and $3. So, in this problem, you first subtract the $3 from the $92. What is $92 - $3? This equals $89. Now, you can subtract the $40 from the $89. What is $89 - $40? That equals $49. This tells you that you will have $49 left in your piggy bank after buying your game.

Example

Let's look at another problem.

Susie scored an 89 on her first test and 92 on her second test. What is her total test score so far?

To solve this problem, you need to add the two scores together. Using mental math, you break apart each number into their respective parts. The 89 breaks up into 80 and 9. The 92 breaks up into 90 and 2. Now you can go ahead and up their matching parts. You add 80 and 90. What does that equal? It equals 170. Next, you add 9 and 2. What does that equal? 11. Now, add up your separate sums. Your answer is 170 + 11 = 181. Susie's total is 181.

Lesson Summary

Let's review what you're learned now. Mental math is about performing math calculations in your head without a calculator or paper. Using mental math with addition involves breaking up the numbers into its separate parts and then adding up the matching parts and then adding up the sums to find the answer. Using mental math with subtraction involves breaking up the number being subtracted into its parts and then subtracting the parts from the number you are subtracting from starting with the smallest part.

Learning Outcomes

Students completing this lesson should have the confidence to:

Define what mental math is

Describe how mental math can be used in addition and subtraction problems

Solve addition and subtraction problems using mental math

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