Mental Math: 10 More, 10 Less - First Grade Mathematics

Did you know that you can add ten or subtract ten from almost any number in your head without counting on your fingers? That mental superpower is called finding 10 more and 10 less. Because our number system groups quantities by tens, adding ten or subtracting ten changes only one single digit in the entire number: the tens digit! The ones digit stays completely untouched. In this chapter, you will master the mental math tricks for finding 10 more and 10 less quickly and confidently.

The Secret Rule of 10 More

Finding 10 more means adding one group of ten to a number (+10).

Finding 10 More:
Start with:      34   (3 Tens, 4 Ones)
Add 1 Ten:     + 10   (1 Ten,  0 Ones)
---------------------------------------
New Number:      44   (4 Tens, 4 Ones)

Notice: The 3 changed to 4. The 4 in the ones place DID NOT CHANGE!

How to Think It in Your Head

To find 10 more than any number, look at the tens digit and increase it by 1. Keep the ones digit exactly the same! 10 more than 27 is 37. 10 more than 51 is 61. 10 more than 82 is 92. It is as fast as counting: 2, 3... 5, 6... 8, 9!

The Secret Rule of 10 Less

Finding 10 less means taking away one group of ten from a number (-10).

Finding 10 Less:
Start with:      68   (6 Tens, 8 Ones)
Subtract 1 Ten:- 10   (1 Ten,  0 Ones)
---------------------------------------
New Number:      58   (5 Tens, 8 Ones)

Notice: The 6 changed to 5. The 8 in the ones place DID NOT CHANGE!

How to Think It in Your Head

To find 10 less than any number, look at the tens digit and decrease it by 1. Keep the ones digit exactly the same! 10 less than 75 is 65. 10 less than 43 is 33. 10 less than 99 is 89.

Seeing 10 More and 10 Less on the 120 Chart

The 120 chart makes 10 more and 10 less visually obvious through vertical movement.

Vertical Motion on the Grid:

          [ 46 ]  <- 10 LESS (Move 1 Step UP)
             ^
             |
          [ 56 ]  <- STARTING NUMBER
             |
             v
          [ 66 ]  <- 10 MORE (Move 1 Step DOWN)

One Step Up, One Step Down

On any standard number chart, moving one step straight down lands on 10 more. Moving one step straight up lands on 10 less. You never have to recount by ones; just take one vertical hop!

Practice Exercises

Exercise 1: What is 10 more than 45? What is 10 less than 45?

Exercise 2: Find the missing numbers in these mental math equations: 63 + 10 = __ 81 - 10 = __ 29 + 10 = __ 74 - 10 = __

Exercise 3: Explain why the ones digit does not change when you find 10 more or 10 less of a number.

Exercise 4: You have 38 building blocks. Your friend gives you a full rod of 10 blocks. How many blocks do you have now? Which digit changed?

Solutions and Explanations

Solution to Exercise 1: 10 more than 45 is 55 (increase the tens digit from 4 to 5). 10 less than 45 is 35 (decrease the tens digit from 4 to 3). The ones digit (5) remains the same in both answers.

Solution to Exercise 2: 63 + 10 = 73 81 - 10 = 71 29 + 10 = 39 74 - 10 = 64

Solution to Exercise 3: The ones digit does not change because the number 10 has a zero in the ones place (10 = 1 ten and 0 ones). When you add or subtract 0 ones from the existing ones, the ones quantity remains identical.

Solution to Exercise 4: You now have 48 blocks (38 + 10 = 48). The tens digit changed from 3 to 4 because you added 1 ten, while the ones digit stayed at 8.