How did estimate you place the decimsl point in a product correctly.

After you have multiplied but you have all these numbers, look to see how many decimal places the numbers you have multiplied.
Lets say you multiply:
     23.45
   x    2.1
= 49.245
Because your first number had 2 decimal places and your second number had 1 decimal place you add that and that is how many decimal places your product has. In this case there is three.