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Mental Addition and Subtraction of 10s or 100s

2.NBT.8
Answer Keys Here

Aligned To Common Core Standard:

Grade 2 Base Ten - 2.NBT.8

When we begin to make calculation in our heads without writing anything down or using a calculator of some kind, we call this mental math. This is a very advantageous skill to have. It can help you determine if the price you are paying for something is a good deal for you. It can also tell you if you received proper change from a purchase you made with a cashier. This skill also tells us when to travel and helps us arrive on time or early to anything we may be responsible to attend. When we have achieved a good deal of skill with mental math it is sign that we are reaching a mastery level of number sense. In this series of worksheets, we begin to explore working with the mental math perspective of increasing the tens and hundreds place by one. This is a great place to start of because it is a skill most students will pick up quickly. This collective of worksheets has students practice the mastery of mental math facts.

Mental Addition and Subtraction Worksheets And Lessons


  • Step-by-step Lesson- I broke out the numbers line to help us. We move between the addition and subtraction of 10s.

  • Guided Lesson - The focus is on on the movement between tens place again here.

  • Guided Lesson Explanation - Sorry, I might have gone a bit overboard with explaining here. I just wanted to make sure a students wouldn't need any outside help.

  • Practice Worksheet - Ten problems that all use a numbers line to prompt the students.

  • Matching Worksheet - Match the missing numbers line spot to the number that is missing.


Homework Sheets

Fill in the partially missing parts of the numbers line.

  • Homework 1 - Complete the number lines below.
  • Homework 2 - We give you answer key that is fully worked out for you.
  • Homework 3 - We focus on the tens place with these.



Practice Worksheets

You will start with blank number lines; except for the start and finish.

  • Practice 1 - Find the final value of the following problem.
  • Practice 2 - You will need to determine the logic of what each number flow presents you with.
  • Practice 3 - You can figure this out mentally and see what fits best for you.



Math Skill Quizzes

Remember to make a mental note of the type of operations that are going on here.

  • Quiz 1 - We fit ten of these problems on the page. Be aware that the answer key carries over 2 pages.
  • Quiz 2 - The focus on this quiz is to subtract.
  • Quiz 3 - We go on the inverse in quiz 3 and focus on adding.


Tips for Mental Addition and Subtraction

Being able to do math accurately without the help of pencil, paper, or a calculator is very empowering. It will help you make confident decisions quicker and more precisely. Gathering all the praise by yourself for doing mental math is just awesome, isn't it? To be able to solve calculations between two and three-digit numbers within a few minutes without having to use a calculator not only boosts your confidence, but it also amplifies your comprehension too. Mental math is associated with number sense, which is the skill to manipulate numbers in your head in one or more ways to solve simple and complex problems. Reportedly, having number sense has an impact on the achieving grasp over algebra. Which is very logical, the greater your understanding of the meaning of variables in an equation, the easier it is to manipulate them to your liking.

A few tricks to do mental math are:

1. They Are Inverse - Remember that these operations are inverse of one another. You can use this to your advantage when working with large values. For example, if you were trying to calculate the value of 294 + 460 you could start by rounding. We could round that simply to 300 + 460. All this would require is for us to add 6 to 294. Since we added 6 to our problem our final solution will be this rounded sum subtracted by 6. So, it makes sense that our end value will be: 300 + 460 = 760 - 6 = 754


2. Learn +1 double - Memorizing sum of doubles is a great way to get a grasp over mental mathematics. By the sum of doubles, we mean; 7 + 7, 10 + 10, 30 + 30, etc. This way, you can quickly add one more to the answer if you get to solve a 'double plus one more,' for a sum 7 + 8, add 7 + 7, then add one more to the answer.


3.Subtract by adding- This trick is based on the connection between addition and subtraction. To find out, find 8 − 6, think, "6 plus how much more makes 8?"
6 + ___ = 8.
This is going to be your answer for 8 – 6 too.


4. The '9-trick.'- If you must add 9 to any number, the trick is to add 10, get the answer, and then instantly minus 1 in your head. You have your final answer. Do the same for subtraction, but you would reverse everything.