Enumerating paragraph with manual number

I have just started using Latex today and I have been looking into how to manually number the paragraphs inside enumeration. I am writing up the solutions to the problems which are not in consecutive order; what I am trying to get looks like the following:

Chapter 8
8.3  Let F = 492 N, x = 33, y = 42
Since we know the magnitude...
(a)  ...
...
(b)  ...
...

8.12 We know the resultant of two vectors ...
A graphical sketch of ...
+--------+
|        |
| Figure |
|        |
+--------+
8.15 ...


In the above representation, I would like to have the ability to:

• manually number the problems of each enumerated paragraph (8.3 -> 8.12 -> 8.15)
• and to indent each of the whole paragraph as shown above

I have tried the following code but it automatically numbers each item for me in an increasing order. Could anyone point me how to manually number each item/each paragraph?

\begin{enumerate}
\item
Let $F = 492 N, x = 33, y = 42$\\
Since we know the magnitude...

\item ...

\item ...
\end{enumerate}


You can use the optional argument of \item:

\documentclass{article}

\begin{document}
\begin{enumerate}
\item[8.2]
Let $F = 492 N, x = 33, y = 42$\\
Since we know the magnitude...

\item[8.12] ...

\item[8.15] ...
\end{enumerate}
\end{document}