\[\boxed{\mathbf{13.}}\]
\[\textbf{а)}\ 50 + (18 + 5) = 50 + 23 =\]
\[= 73\]
\[62 - (12 - 7) = 62 - 5 = 57\]
\[\textbf{б)}\ 92 - (27 - 24) = 92 - 3 =\]
\[= 89\]
\[(34 + 17) - 15 = 51 - 15 = 36\]
\[64 + (91 - 55) = 64 + 36 =\]
\[= 100\]
\[(18 + 44) - 62 = 62 - 62 = 0\]