Estimation is a quick way to work out the approximate answer to a math problem. Estimation is a great way to check your work. To estimate, you need to know how to round numbers. Once you learn to round numbers and estimate, you can do more complex math problems in your head.
Estimating is an important part of math and very handy for everyday life – for making sure you are paying the right amount of money when you go shopping , or for working out lengths of time, distance, weight, etc.
For example, say you are adding 527 + 327 + 91 + 83 + 411
This is a pretty complex sum and one that will take time to solve. If you need to work out the rough (or estimate) answer quickly, or if you want to cross-check that your final calculations were correct, you can make this addition easier by rounding each number to the nearest hundred and then adding them:
500 + 300 + 100 + 100 + 400 = 1,400
The approximate answer is 1,400
which isn’t far off the exact answer: 1,439.
The same goes for multiplication. Say you want to multiply
511 x 403
You again round the numbers to the nearest 100:
500 x 400 = 200,000
Not far off the exact answer: 205,933.
But watch out! You have to consider how you are rounding the numbers. Your estimations can quite wrong. You’ll arrive at bad estimates when you:
- Round numbers that are close to the middle of the range, such as 349 or 562.
- Round too many numbers in the same direction (be it up or down)
- Multiply or divide rounded numbers (Generally, multiplying or dividing throw off estimates more than addition or subtraction)
Let’s say you are multiplying 242 X 437
You round both numbers:
200 x 300 = 60,000
However, the exact sum is 105,754 – a sum that is of by approximately 45,000, 45,754 to be exact.
Not a very good estimate. What went wrong? Both numbers are close to the middle range, so when you round them down it changes their values by a lot. Rounding the numbers down, therefore lowered their values by a lot.
If you are interested in learning to round numbers, we have some free worksheets on rounding numbers and place value that you can try out on these pages: grade 2, grade 3, grade 4 and grade 5.
