Let
x--------> the number
we know that
1) the phrase, "the sum of 12 and a number"
represent the expression --------> [tex](12+x)[/tex]
and
2) the complete phrase, "one third the sum of 12 and a number"
represent the expression --------> [tex]\frac{1}{3}(12+x)[/tex]
therefore
the answer is
[tex]\frac{1}{3}(12+x)[/tex]