Using the averaging method, find the threshold of the $4 \times 4$-bit image. Let the initial value of the threshold be the average of the gray levels of the image. The iteration stops when the difference between two consecutive threshold values becomes less than 0.5 .
$$
\left[\begin{array}{rrrr}
140 & 10 & 5 & 6 \\
74 & 2 & 7 & 6 \\
21 & 5 & 6 & 5 \\
2 & 6 & 5 & 5
\end{array}\right]
$$