It's easier to work with rounded numbers because they end in zero (or for big numbers, lots of zeroes). The trick is to find a number with zeroes that it's very close to.
Think of it this way. If you wanted to figure out how much money it would cost to buy each of your 18 classmates a pack of pencils for 99 cents, would you rather multiply:
18 x $0.99
or
18 x $1?
Answer in the comments below!
To practice estimating, take a look at this estimating game! In this game, you will click on the number line where you think the answer is. For example, the answer to 530+149 is between 500 and 600, so I'd click there. To start out I would recommend you choose either:
* Add tens
* Add hundreds
* Subtract tens
* Subtract hundreds