It'll make it easier to multiply. If you round 3.9 to four, and 5.3 to five, it's quite easy to figure out the answer is about 20. I hope this helps! :)
You can round these numbers to the closes whole number; 3.9-->4 and 5.3-->5, and then you can do 5x4 to get 20; and if you multiply regularly without estimating, your answer will be around 20.
