Estimation as a check

I am learning to check any addition and subtraction problem I cannot solve mentally by estimating the answer.

You may have used a calculator or an algorithm

Alex had $479 and he spent $138. How much money does he have left? 479 - 138 = ?

Below are three potential answers. Let’s use our estimation skills to work out which ones cannot be correct.

341

617

381

This one can’t be the answer. If Alex started with $479 and spent money he can’t end up with more than what he started with. It looks like they may have added here rather than subtracted.

Alex started with $479 and spent more than $100. I know $479 - $100 is $379. This answer is more than that so it can’t be correct.

This one is the correct answer.

A ping pong ball factory made 486 balls in the morning and 473 in the afternoon. How many balls did they make in total? 486 + 473 = ?

Below are three potential answers. Let’s use our estimation skills to work out which ones cannot be correct.

946

1008

959

This one can’t be the answer. Both the morning amount and the afternoon amount were less than 500. I know 500 + 500 = 1000 so the correct answer must be less than this.

This is the correct answer.

I can see that 486 has 6 in the ones column and 473 has 3 in the ones column. If I add the ones together I get 9. So the answer I’m looking for must have a 9 in the ones column. This can’t be the correct answer.

Now try these questions on your own. Three answers have been provided for each question. Use your estimation skills to choose the correct one.

Need some more practise? Try Pg61 of Teacher Tools Addition and Subtraction Book Numeracy resources

1) 248 + 247 = 495, 501, 1

2) 799 + 214 = 102, 1002, 1013

3) 843 – 149 = 992, 694, 444

4) 687 – 214 = 473, 901, 494

5) 518 + 2,009 = 727, 2,527 2,509

Click again when you have completed each question and the answer will be displayed.