How does an estimate you place the decimal point when multiplying 3.9 and 5.3

Well, Mathematicians are lazy and they don’t want to have to multiply “large numbers” so they round them to make their life easier. For example: 3.9 rounds to 4, and 5.3 rounds to 5, do just multiply 4 and five.

3.9= 4
5.3= 5

I don’t know if that’s correct