What is the key point of vertical calculation of decimal multiplied by integer?

What is the key point of vertical calculation of decimal multiplied by integer?

Calculate by integral multiplication
The point is that the number of decimal places is equal to the sum of the decimal places of two multipliers
For example, the result of 2.31 * 3.42 must be 4 decimal places
If the last bit is 0, 0 must also be counted
For example, if 2.5 * 4 is written as 10.0 according to one decimal place, the 0 after the decimal point will be omitted after calculating the place occupation