Answer
$5$
Work Step by Step
Let $x$ be the number.
The conditions of the problem translate to the equation
\begin{array}{l}\require{cancel}
\dfrac{12+x}{41+2x}=\dfrac{1}{3}
.\end{array}
By cross-multiplication and using the properties of equality, the equation above is equivalent to
\begin{array}{l}\require{cancel}
(12+x)3=(41+2x)1
\\\\
36+3x=41+2x
\\\\
3x-2x=41-36
\\\\
x=5
.\end{array}
Hence the number is $
5
.$
