Fix table cells width and alignment

Question

hope you all feeling well and safe

please help with the code below, the issue is with the cell width of last 3 column (3rd row onward), I want them to have similar width. please find attached code and picture

\begin{table}[t]
    \centering
    \caption{Ranges and levels on independent variables}
    \label{tab:activation ranges}
    \begin{tabular}{|c|c|c|c|}
        \hline
        \multirow{2}{*}{\textbf{Independent Variable}} & \multicolumn{3}{c}{\textbf{Ranges and Coded Levels}} \\ \cline{2-4} 
        & \textbf{-1}      & \textbf{0}      & \textbf{+1}     \\ \hline
        Acid concentration (\%)                        &25               & 55              & 85              \\ \hline
        Temperature (°C)                               & 45               & 80              & 115             \\ \hline
        Residence time (min)                           & 60               & 120             & 180             \\ \hline
    \end{tabular}
\end{table}

Answer 1

One can measure the width of the string \textbf{Ranges and Coded Levels} and set the widths of columns 2 thru 4 to take up (roughly) one third of that width.

A separate recommendation: Do get rid of all vertical rules, and do replace the basic \hline and \cline directives with fewer but better-spaced lines using the macros of the booktabs package: \toprule, \midrule, \cmidrule, and \bottomrule.

\documentclass{article}
\usepackage{booktabs}
\usepackage{array} % for 'w' column type
\newlength\mylen
\settowidth\mylen{\textbf{Ranges and Coded Levels}}
\addtolength\mylen{-4\tabcolsep}
\setlength{\mylen}{\dimexpr\mylen/3\relax}

\begin{document}
\begin{table}[t]
    \centering
    \caption{Ranges and levels on independent variables\strut}
    \label{tab:activation ranges}
    
    \begin{tabular}{@{} l *{3}{w{c}{\mylen}} @{}}
    \toprule
    \textbf{Independent Variable} & 
    \multicolumn{3}{c@{}}{\textbf{Ranges and Coded Levels}} \\    
    \cmidrule(l){2-4} 
    & $\mathbf{-1}$ & \textbf{0} & $\mathbf{+1}$ \\ 
    \midrule
    Acid concentration (\%) & 25 &  55 &  85 \\ %\hline
    Temperature (°C)        & 45 &  80 & 115 \\ %\hline
    Residence time (min)    & 60 & 120 & 180 \\ 
    \bottomrule %\hline
    \end{tabular}
\end{table}
\end{document}

Answer 2

I propose this layout with siunitx, so the numbers be right-aligned, and a well-chosen column-width. I added some improvements, such as loading caption to have a more sensible verical space between caption and table, and the first column left-aligned, which looks better in my opinion:

\documentclass{article}

\usepackage[table]{xcolor}
\usepackage{booktabs}
\usepackage{tabularx, multirow, caption, siunitx}
\usepackage{makecell}

\begin{document}

\begin{table}[t]
\setlength{\extrarowheight}{2pt}
\sisetup{table-format=3.0, table-number-alignment=center, table-column-width=12mm}
\captionsetup{skip=4pt}
\centering
\caption{Ranges and levels on independent variables}
\label{tab:activation ranges}
\begin{tabular}{|l|S|S|S|}
    \hline
    \multirow{2}{*}{\textbf{Independent Variable}} & \multicolumn{3}{@{}c@{}|}{\textbf{Ranges and Coded Levels}} \\ \cline{2-4}
    & {$\mathbf{-1}$} & {\textbf{0}} & {$ \mathbf{+1} $} \\ \hline
    Acid concentration (\%) &25 & 55 & 85 \\ \hline
    Temperature (°C) & 45 & 80 & 115 \\ \hline
    Residence time (min) & 60 & 120 & 180 \\ \hline
\end{tabular}
\end{table}

\end{document}