Mr. Gifford’s 5 th Grade Math Lesson #10 Mental Math using values of 10.

One of the objects of understanding math is to be able to do math quickly without the use of tools. We call this Mental Math when we use our brains to figure out numbers. Mental math is a part of math that is not learned overnight but progressively gets better as you PRACTICE!

The first part of mental math we will look at is working with numbers that you know. 10 is a great number to begin with. Let’s practice counting by 10’s 10…20…30…40…50…and so on

Now let’s add a digit 0 to the end of those numbers 100…200…300…400…500…and so on. If we add another digit 0 to the end of the numbers we can continue the pattern. 1,000…2,000…3,000…4,000…5,000 …and so on.

So, now if you want to add them it is easy. First take the 0’s away Now, just add the digits.

Finally, add the 0’s back into the number for the final answer. 600

Let’s look at other numbers With addition, you can add ANY digits in Any order. This is called the COMMUNATIVE PROPERTY = = = 56

Here is the Compensation Mental Math Method. Let’s add Mentally you think: Add 1 to 49 = 50 Now add = = 8 68 Take 1 away from 68 because you added 1 to the 50 at the beginning. Final answer is 67.

Let’s Practice Mentally add Take = 50 Then = = 61

Add this Add 2 to 48 to make = = 7 67 Take 2 away from 67 because you added 2 to 48 at the beginning. The Final answer is 65.

You can do the same with subtraction. 56 – 13 = 50 – 10 = 40 6 – 3 = 3 43

Your Turn to Practice Add 1 to 69 = = = = 3 93 Take 1 away from 93 because you added 1 at the beginning. Final answer = 92

Try this = = =

Here is another 68 – – 20 = 40 8 – 3 =

Here is a tough one: = Turn 249 into 250 And Turn 9 into = = is your answer. Now take the 2 digits away that you added at the beginning to get 262 for your final answer.

Now you can practice by yourself. Mental Math (Remember to use DR. QUFOSOAC) Show how you can use mental math to add or subtract = =

*Answer questions 3-4 using the following data. National Monuments Name State Acres George Washington Carver Missouri 210 Navajo Arizona 360 Fort Sumter South Carolina 200 Russell Cave Alabama How many more acres are there at Navajo monument than at George Washington Carver monument? 4. How many acres are there at Fort Sumter and Russell Cave combined?

5. Fresh Market bought 56 lb of apples in August from a local orchard. In September, the market purchased an additional 52 lb of apples and 32 lb of strawberries. How many pounds of fruit did the market buy? A 108 lb B 140 lb C 150 lb D 240 lb

Good work today. Continue using mental math every opportunity that you get. Thank you for joining me and I will see you next time.