Answer:
Both type of numbers have a decimal point but the decimal numbers can contain digits other than 0 so you have to worry for these numbers when you're multiplying and the placement of the decimal point in the answer.
A way think about it is sum the total numbers after the decimal point, this work because the numbers can be represented as fractions (explanation below) so If one factor has 2 decimal places and another factor has 1 decimal place the answer have 3 decimal places.
Explanation:
A whole can be represented as 2 and so on, while a decimal number is represented as 3.14 when the numbers after the decimal point can be different of 0.
So when you multiply two decimal numbers check the total number of number after the decimal point and sum these amounts.
Where the total number of numbers after the decimal point is equal to the power of 10 in the denominator that's mean equal to 3. However this method takes more time, so is more simply to say that the total of numbers after the decimal point is the sum of the total of numbers after the decimal point of each decimal number.