<h2>
Answer:</h2>
The expression is given by:
18+n
where n is a number.
<h2>
Step-by-step explanation:</h2>
We are asked to represent the given word phrase as a mathematical expression.
The word phrase is given as follows:
The sum of 18 and a number.
Let the unknown number be represented by a variable n.
Now, we know that the sum of 18 and n is such that both the terms 18 and n are separated by a addition sign.
Hence, the expression is: 18+n
