Lesson video

Hello, my name's Miss Parnham.

In this lesson, we will be learning how to estimate answers.

Let's have a think about which of the following calculations will be the most appropriate to estimate the answer to 87 multiplied by 28.

I wonder if you got it right.

The convention when we're estimating is always to round to one significant figure.

So 87 rounded to one significant figure is 90 and 28 rounded to one significant figure is 30.

That is why 90 multiplied by 30 is the best estimate here for the answer to the calculation 87 multiplied by 28.

Let's estimate the answer to 32 multiplied by 843 by rounding each of them to one significant figure.

So this approximates to 30 multiplied by 800, which is very easy to do mentally, and this gives us 24,000.

Now, is this an underestimate or an overestimate of the real answer to 32 multiplied by 843? Well, if we look 32 rounded down to 30 and 843 rounded down to 800, so by multiplying two lower numbers, we would get an answer that is lower than the true answer to 32 multiplied by 843.

So in this case, we know it is an underestimate.

Let's use some slightly different skills with this problem.

How could we estimate route 43? Knowledge of square numbers will help here.

Route 43 must be somewhere between route 36 and route 49.

Why did I choose those routes? Because route 36 is six and route 49 is seven.

So route 43 is somewhere between these two.

So the answer lies in between six and seven.

Here's some questions for you to try.

Pause the video to complete the task and restart the video when you're finished.

Here are the answers.

If we focus on part "e" of this question, then 47 would round 50 and 478 to 500.

So we would have 2,500 subtract 500 on the top of that division.

This gives us 2000 and we would divide that by 0.

5.

Now dividing by 0.

5 causes a number to double.

So that's where the solution of 4,000 has come from.

Let's estimate the answer to a multi set problem by first rounding every number in the problem to one significant figure.

Then we can work out 300 add 100 and we can work out four multiplied by five to give us a simplified division.

And then finally 400 divided by 20, which is 20.

So an estimate for the solution to this calculation is 20.

Here's some questions for you to try.

Pause the video to complete the task and restart the video when you're finished.

Here are the answers.

In part "a" the numbers in the fraction are already given to one significant figure.

So we only needed to round 136 to 100 before calculating.

Whereas in part "e", we need to remember the 11 must be rounded to 10 before we work out three tenths of 400 and get the answer of 120 pounds.

Here's some more questions for you to try.

Pause the video to complete the task and restart the video when you're finished.

Here are the answers.

In question three, one of the numbers was rounded up and one of the numbers was rounded down.

So it wouldn't be easy to tell whether the 600 pounds is an overestimate for the cost of the carpet or an underestimate.

Here's a further question for you to try.

Pause the video to complete the task and restart the video when you're finished.

Here are the answers.

In part "b" we have a cube as indicated by the markings on the side showing that they're all 3.

2, so that number rounds down.

So we're multiplying a lesser number to get 27.

So we know that 27 is an underestimate for the true volume of that cube.

That's all for this lesson.

Thank you for watching.