\[\boxed{\mathbf{6.}}\]
\[30 + 8 = 38\]
\[38 - 8 = 30\]
\[38 - 30 = 8\]
\[40 + 6 = 46\]
\[46 - 6 = 40\]
\[46 - 40 = 6\]
\[50 + 3 = 53\]
\[53 - 3 = 50\]
\[53 - 50 = 3\]
\[90 + 9 = 99\]
\[99 - 9 = 90\]
\[99 - 90 = 9\]