1-646-564-2231

Using Mental Math For Estimation

Key Concepts

• Mental subtraction
• Rounding numbers to estimate
• Using front-end estimation.

Introduction

In this chapter, we will learn about 2-digits addition and subtraction of numbers, and round the numbers up to the nearest estimation of 10 and add on.

Adding 2-digit numbers mentally using regrouping of numbers and separate their place values.

Example 1:  Add mentally 65 + 32 using number bonds.

Sol:

Step 1: Add 3 tens to 65 => 65 + 30 = 95

Step 2: Add 2 ones to the result = > 95 + 2 = 97

Example: Add mentally 54 + 42 using number bonds.

Sol:

Step 1: Add 4 tens to 54 => 54 + 42 = 94

Step 2: Add 2 ones to the result => 94 + 2 = 96

2.2 Mental Subtraction

Subtracting 2-digit numbers mentally, using the tens and ones by regrouping numbers.

Example: Find 97 – 24

Step 1: Subtract 2 tens from 97 – 20 = 77

Step 2: Subtract 4 ones from the result 77 – 4 = 73

So, 97 – 24 = 73

Example 2:  Subtracting the number using tens, then add the extra one’s strategy.

53 – 28

Step 1: Subtract 30 from 53 => 53 – 30 = 23

Step 2: Add 2 to the result => 23 + 2 = 25

So, 53 – 28 = 25

Adding two-digit numbers mentally by adding 100, then subtracting the extra ones.

Example: Find 49 + 97

Sol: Add 100, then subtract the extra ones

Step 1: Add 100 to 49 => 100 + 49 = 149

Step 2: Subtract 3 from 149 => 149 – 3 = 146

So, 49 + 97 =146

Example: Find 95 + 96

Sol: Add the hundreds, then subtract the extra ones

100 + 100 = 200

Step 2: Subtract 5 and 4 from 200

200 – 5 – 4 = 191

So, 95 + 96 = 191

2.4 Rounding Numbers to Estimate

2.4.1 Round the number:

2576 is 2600 when rounded to the nearest hundred.

Estimate:  A number close to the exact number.

Reasonable: An estimated answer is reasonable when it is close to the actual answer.

Example: Round 550 to the nearest 100?

Sol:

Example: Find 405 + 520 round to the nearest hundred, then add.

Sol:  405 + 520 = 925

405 rounds to 400

520 rounds to 500

400 + 500 = 900

Example: Find 869 – 214 rounds to the nearest hundred, then subtract.

Sol:  869 – 214 = 655

869 rounds to 900

214 rounds to 200

900 – 200 = 700

2.5 Using Front-End Estimation

Front-end Estimation: Estimation that uses leading digits to estimate the sum and the digit.

Example: Find the sum of 478 and 96 using front-end estimation?

Sol:

Example: Find 465 + 216 use the value of leading digits to add.

Sol:  465 + 216 = 681

Example: Find 669 – 114. Use the value to the leading digits to subtract.

Sol: 669 – 114 = 555

Exercise:

1. Find 26 + 31.
2. Find 28 + 34.
3. Find 46 + 56.
4. Subtract 33 – 23.
5. Subtract 63 – 53.
6. Add 98 and 97, by adding the hundreds and subtracting the ones.
7. Find 34 + 26.
8. Find 465 + 519 and round to the nearest hundreds.
9. Find 679 – 413 and estimate the nearest value.
10. Find 98 – 33.
11. Subtract mentally 65 – 37.
12. Find the sum (or) difference and round up the answer of 234 and 457.
13. Find 36 + 26.
14. Find 715 – 232.
15. Jack has $66. He saves another$46. Estimate the amount he is having with him.

What we have learnt:

• Subtract mentally
• Add or subtract, then use rounding to check that their answers are reasonable.
• Use front-end estimation to check their sums and differences.

More Related TOPICS

Use Models to Subtract Mixed Numbers

Key Concepts Using models to subtract mixed numbers Subtract mixed numbers Addition and subtraction of mixed

Use Models to Add Mixed Numbers

Key Concepts Using models to add mixed numbers Adding mixed numbers Introduction  In this chapter, we

Find Common Denominators

Key Concepts Equivalent fractions and common denominators Finding common denominators Introduction In this chapter, we will