\[\boxed{\mathbf{15}\mathbf{.}}\]
\[\textbf{а)}\ 42 + 4 + 1 = 46 + 1 = 47\]
\[5 + 23 + 1 = 28 + 1 = 29\]
\[39 - 2 - 1 = 37 - 1 = 36\]
\[68 - 6 - 1 = 62 - 1 = 61\]
\[\textbf{б)}\ 100 - 1 - 5 = 99 - 5 = 94\]
\[89 - (34 - 30) = 89 - 4 = 85\]