What you need to know

Things to remember:

  • Estimating is just rounding
  • If the units are 4 or less, we round down.
  • If the units are 5 or more, we round up.
  • We use estimates to see if our answer is close to the correct one.

Let’s have a look at some quick rounding questions.

Number           Rounded

22                      20

31                      30

65                      70

38                      49

We will use estimating in a question like the following.

“Katie says that 16+31=77. Use estimate to see if she is correct.”

We will do this in two steps.

Step 1: Round both numbers

Number           Rounded

16                         20

31                        30

Step 2: Add or subtract your numbers.

Katie was doing an addition, so we need to add these two numbers.

20+30=50

We can see that Katie’s answer should have been closer to 50, so she wasn’t right by saying 77. (The actual answer is 47).

Let’s see how we do it with subtraction. Find an estimation for the following subtraction:

55-28= \_\_

60-30= 30

So, we do it in the same way, we just have to subtract instead of add!

Example Questions

Question 1: Find an estimate of the following addition:

23 + 62 = \_\_

Answer

Remember we need to round our numbers first.

 

Number           Rounded

23                    20

62                    60

 

And then add them.

 

20 + 60 = 80

 

So, our estimate is 80.

Question 2: Find an estimate of the following addition:

76 - 13 = \_\_

Answer

Remember we need to round our numbers first.

 

Number           Rounded

76                    80

13                    10

 

And then subtract them.

 

80 - 10 = 70

 

So, our estimate is 70.

 

Is this a good estimate?