hspace in tabular

For proper spacing in your "equation" setup, you need to use something like @{${}={}$} (in order to make it a true relation). I've updated your tabular to use this, as well as inserting the total:

\documentclass{article}
\begin{document}
\begin{tabular}{ccr@{${}={}$}r} 
  $X_j$ & $f_j$ & \multicolumn{1}{c}{$f_jX_j$} \\
  \hline
  28 & 1 & (1)(28) & 28 \\
  27 & 0 & (0)(27) &  0 \\
  26 & 1 & (1)(26) & 26 \\
  25 & 2 & (2)(25) & 50 \\
  24 & 3 & (3)(24) & 72 \\
  23 & 4 & (4)(23) & 92 \\
  22 & 3 & (3)(22) & 66 \\
  21 & 0 & (0)(21) &  0 \\
  20 & 1 & (1)(20) & 20 \\
  19 & 2 & (2)(19) & 38 \\
  18 & 1 & (1)(18) & 18 \\
  17 & 0 & (0)(17) &  0 \\
  16 & 1 & (1)(16) & 16 \\
  15 & 0 & (0)(15) &  0 \\
  14 & 1 & (1)(14) & 14 \\
  \hline
  & $n=20$ & $\displaystyle\sum^{k}_{j=1}f_jX_j$ & $440$
\end{tabular}
\end{document}​

You can add some padding before the third column and hide the width of the big summation:

\documentclass{beamer}
\usepackage{array}
\begin{document}
\begin{frame}
\begin{tabular}{cc@{\hspace{4em}}r@{}>{${}={}$\hfill}r}
$X_j$&$f_j$&\multicolumn{1}{c}{$f_jX_j$}\\
\hline
28&1&(1)(28)&28\\
27&0&(0)(27)&0\\
26&1&(1)(26)&26\\
25&2&(2)(25)&50\\
24&3&(3)(24)&72\\
23&4&(4)(23)&92\\
22&3&(3)(22)&66\\
21&0&(0)(21)&0\\
20&1&(1)(20)&20\\
19&2&(2)(19)&38\\
18&1&(1)(18)&18\\
17&0&(0)(17)&0\\
16&1&(1)(16)&16\\
15&0&(0)(15)&0\\
14&1&(1)(14)&14\\
\hline
&$n=20$&\hidewidth$\displaystyle\sum^{k}_{j=1}f_jX_j=440$\hidewidth\hfil
\end{tabular}
\end{frame}
\end{document}

Experiment with the size of the spacing until the result is satisfying