# Checking reasonableness of calculations

Lesson

## Is it reasonable?

Checking if an answer is reasonable means we check to see whether an answer is a good approximation or estimate, for our problem. This is really important because we can see if the estimate makes sense, or, we may not need the exact answer, but an estimate will do

### How to estimate

When we estimate, there are some methods that are really useful, and we'll explore some of these in our video, including:

• rounding one or both of our numbers
• thinking about how multiplication and division are related
• thinking about what happens when we add or subtract

In this video, we'll work through a few examples, and also estimate one problem two different ways.

Remember!

When we estimate, we are not after an exact answer, but an answer that gives us a good idea if our answer is correct. We can use different ways to estimate our answer, so having a few ideas to choose from is a great help.

#### Worked Examples

##### Question 1

Is $4000$4000 a reasonable estimate for $42\times8$42×8?

1. Yes

A

No

B

Yes

A

No

B

##### Question 2

Is $400$400 a reasonable estimate for $2002\div5$2002÷​5?

1. No

A

Yes

B

No

A

Yes

B

##### Question 3

Irene had $120$120 dolls and she gave $18$18 to her friend Charlie. Charlie said that Irene had $138$138 dolls left. Is Charlie's statement reasonable?

1. Yes

A

No

B

Yes

A

No

B

### Outcomes

#### NA3-1

Use a range of additive and simple multiplicative strategies with whole numbers, fractions, decimals, and percentages.