How would you write 20 milliseconds as decimal?

A decimal number is simply a way of representing a number in such a way that the place value of each digit is ten times that of the digit to its right. It may or may not contain a fractional part. If not, the decimal representation does not require a decimal point. So the decimal for 20 ms is 20 ms. If you want it as a decimal fraction of some other quantity then you need to specify that second quantity.