Mental Math: Addition and Subtraction

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  • 0:05 Mental Math
  • 0:49 Addition
  • 2:04 Subtraction
  • 3:10 Example
  • 4:02 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

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.


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.


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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