Lesson10- Estimation of Decimals

Objective

At the end of this lesson, students should be able to estimate the answers to questions involving addition and subtraction of decimals.

An estimate is a rough guess. In this lesson, we will learn how to estimate decimals in addition and subtraction questions.

To estimate a decimal, you may quickly round it to the nearest whole number, tenth, hundredth or thousandth before adding or subtracting.

Example 1

Estimate the following decimals.

a) 4.6 + 2.1                     b.  5.6 – 3.5

Solution

a) 4.6 + 2.1 = 6.7

Estimate = 5+2 = 7

b)5.6 – 3.5= 2.1

Estimate = 6 – 4 = 2

Example 2

Estimate the following decimals(leave your answer to 1 decimal place)

a)6.49 + 1.25                           b.  9.35 – 4.35

Example 3

Estimate to the nearest hundredth.

a) 13.725 + 3.123                       b.  15.123 – 12.281

Solution

NB: The decimals are rounded to 2 decimal places before adding or subtracting.

a) 13.725 + 3.123 = 16.848

Estimate: 13.73 + 3.12 = 16.85

b) 15.123 – 12.281 = 2.842

Estimate = 15.12 – 12.28 = 2.84