There are many strategies to add bigger numbers! It is smart to estimate before adding. Then you will know “about how much” your sum should be.
The numbers being added together are addends. The answer to an addition equation is the sum.
Estimating a sum will give you an idea of how big your sum will be.
Try to find estimates that are close to the original numbers so your estimate will be fairly accurate.
Numbers that are close to the original numbers but easier to add are called compatible numbers.
One way to find compatible numbers is to round.
295 rounded to the nearest 10 or 100 is 300. That estimate is off by 5 ones.
112 rounded to the nearest 10 is 110. That estimate is off by 2, but in the opposite direction from our other estimate.
All together the estimate is only 3 ones away from the actual sum.
Why Is Estimating Important in Math?
- Helps students check the reasonableness of their answers.
- Builds number sense and mental math skills.
- Simplifies complex calculations during problem-solving.
Different estimation strategies will give you sums that are estimates at different levels of accuracy.
- Rounding to the nearest hundred is usually easier, but less accurate.
- Rounding to nearest ten is usually a little harder but more accurate.
- The accuracy of estimates using compatible numbers depends on how accurate your compatible addends are.
Examples of Estimating in Addition and Subtraction
Addition Example:
Problem: 276 + 129
Estimate: 300 + 130 = 430
Subtraction Example:
Problem: 548 – 322
Estimate: 550 – 320 = 230
Estimating is kind of like using addition and subtraction strategies. You need to choose the strategy you like best, while thinking about the accuracy of your estimate.
Questions to ask yourself as you estimate:
- How accurate does my estimate need to be?
- What could go wrong if my estimate is too low?
- What could go wrong if my estimate is too high?
- How will the numbers I am using to estimate be impacted by rounding?
Practice Problems
Is rounding to the nearest 10 or 100 more accurate?
Omar has $837. He wants to buy a new TV that costs $622. About how much money will he have left?
Round to the nearest 10 to find the sum.
448 + 229 =
Round to the nearest 100 to find the difference.
682 – 545 =
