Question

Explain how estimation helps you to place the decimal point when multiplying 3.9 x 5.3 ?

Answer 1

Well the answer to that is 20.67.
Also, you can simply find this out by placing the decimal as you solve the problem, meaning as when you solve like this
  3.9
x5.3
--------
    .
Drag the decimal down.

Answer 2

If you round 3.9 to 4 and 5.3 to 5 you can easily multiply those numbers together and get 15.