\[\boxed{\mathbf{13}\mathbf{.}}\]
\[\textbf{а)}\ 15 + 0 = 15\]
\[0 + 21 = 21\]
\[13 - 13 = 0\]
\[40 - 0 = 40\]
\[2)\ 4 + 25 - 0 = 29 - 0 = 29\]
\[57 + 0 - 3 = 57 - 3 = 54\]
\[67 - (20 - 20) = 67 - 0 = 67\]
\[48 - (5 + 43) = 48 - 48 = 0\]