B.20 Missing $ inserted

This message can be caused by a number of errors:

You might have missed the beginning of one of the mathematics environments (that is, you've used a command that must only appear in maths mode).
You may have typed $ instead of \$ (you actually want a dollar symbol to appear). Recall from §4.3. Special Characters and Symbols that if you want a $ sign to appear you need to do \$ not just $.
You would have got this error message if you had done, say,
✘ expenditure came to $2000.00
instead of
✔ expenditure came to \$2000.00
You may have missed the end of a mathematics environment, or you may have a paragraph break within an in-line or displayed maths environment, where it is not permitted. Make sure you don't have any blank lines within the environment. If you want a blank line in your code to make it easier to edit, try having a percent sign at the start of an empty line to ensure that the line is ignored by LaTeX. For example:
\begin{equation} % E = mc^2 % \end{equation}

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