13.0 The valid number is 1 3 0.

Let me answer your question,

Follow-up: Your explanation is great. I want to ask you another question. I hope you can use your little wisdom to answer my question.

3.0× 10? Its valid digits are 3 and 0. Is that so? (right)

3.0× 10? Its significant digits are 3, 0, 0, 0, right? (error)

The significant digits of a number are counted from the first digit that is not 0, and then there are several significant digits.

If expressed by scientific counting method, the effective number is only the previous 3.0, instead of 10 after scientific counting method?