How do I fix this overfull warning that is caused by the inline math equation? When I used \sloppy on the paragraph related, underfull warning appeared instead. I've included a MWE below. If it helps, I use lualatex.
\documentclass{report} \begin{document} Firstly, the processing time in the Client-side Mobile Application is calculated by substracting the end time with the start time, such as $ProcessingTime = EndTime - StartTime$ using the help of an API in React Native. \end{document} Here are some possibilities:
Live with over- and underful boxes as long as they are not too bad.
Rephrase the sentence to allow for better line breaks.
Replace inline math by displayed math.
Unrelated: as noted by @Willoughby in the comments, it is better to use \mathit for words in math mode that signify variables.
\documentclass{report} \begin{document} Firstly, the processing time in the Client-side Mobile Application is calculated by substracting the start from the end time, $\mathit{ProcessingTime} = \mathit{EndTime} - \mathit{StartTime}$, using the help of an API in React Native. Firstly, the processing time in the Client-side Mobile Application is calculated by substracting the start from the end time \[ \mathit{ProcessingTime} = \mathit{EndTime} - \mathit{StartTime}\] using the help of an API in React Native. \end{document} 
\(\mathit{ProcessingTime} = \mathit{EndTime} - \mathit{StartTime}\)for better kerning.Version 4.70blatexmk with-pdfluaflag.