3.9 can be rounded to 4.
5.3 can be rounded to 5.
4 x 5 = 20
That tells you that 3.9 x 5.3 has a digit in the tens place.
Then you can proceed to multiply 39 and 53, which equals 2067.
(Note: I removed the decimals on purpose)
Now we know that in 2067, the 2 should be in the tens place. That means 0 is in the ones place and that 6 is in the tenths place. That leaves 7 to be in the hundreths place.
Therefore, 3.9 x 5.3 = 20.67