From the given frequency distribution:
\[
\begin{array}{|c|c|}
\hline
f & x \\
\hline
1 & 42 \\
3 & 43 \\
3 & 46 \\
1 & 51 \\
2 & 52 \\
2 & 54 \\
2 & 55 \\
2 & 56 \\
\hline
\end{array}
\]
We construct the data points by repeating each value \( x \) according to its frequency \( f \):
\[
\text{Data points} = [42, 43, 43, 43, 46, 46, 46, 51, 52, 52, 54, 54, 55, 55, 56, 56]
\]
The sorted data points are:
\[
\text{Sorted data} = [42, 43, 43, 43, 46, 46, 46, 51, 52, 52, 54, 54, 55, 55, 56, 56]
\]
To find the median, we use the formula for the rank of the median:
\[
\text{Rank} = Q \times (N + 1) = 0.5 \times (16 + 1) = 8.5
\]
Since the rank is \( 8.5 \), we take the average of the values at positions \( 8 \) and \( 9 \) in the sorted list:
\[
Q = \frac{X_{\text{lower}} + X_{\text{upper}}}{2} = \frac{51 + 52}{2} = 51.5
\]
The median of the frequency distribution is
\[
\boxed{51.5}
\]