Answer
$$ 15+(1\cdot3)=18$$
Work Step by Step
Orders of operations can be recalled using the acronym PEDMAS. This means that parenthesis are always the first order of operations. This is followed by exponents, division, multiplication, addition, and subtraction, in that order. If there is a tie, we move left to right. In this problem, we add parenthesis so that the order of operations changes so that the number 18 results. Doing this, we find:
$$ 15+ 1\cdot 3 \\ 15+(1\cdot3)=18$$
