\[\boxed{\mathbf{3}\mathbf{.}}\]
\[24 + 12 = 24 + 10 + 2 =\]
\[= 34 + 2 = 36\]
\[36 + 11 = 36 + 10 + 1 =\]
\[= 46 + 1 = 47\]
\[34 + 24 = 34 + 20 + 4 =\]
\[= 54 + 4 = 58\]
\[51 + 32 = 51 + 30 + 2 =\]
\[= 81 + 2 = 83\]
\[65 + 13 = 65 + 10 + 3 =\]
\[= 75 + 3 = 78\]
\[72 + 25 = 72 + 20 + 5 =\]
\[= 92 + 5 = 97\]
\[83 + 12 = 83 + 10 + 2 =\]
\[= 93 + 2 = 95\]
\[36 + 43 = 36 + 40 + 3 =\]
\[= 76 + 3 = 79\]