"A number" means a variable. Pick a letter - x is a popular one. "The sum" means you add, and what you add comes next, 6.

We can write this expression: x + 6

Let's use a different variable, n. Twelve times the number is 12n. "Less" means you subtract one part of the expression from another. Be careful which part is subtracted from which.

One way to write this expression is: 12n - 30

the sum of the girl's age and half the boy's age

There are two variables here, because we don't know the ages of the girl or the boy. Let's call the girl's age x and the boy's age y. The word "sum" means we are adding the two terms. The boy's age needs be divided in half before it gets into the expression.

We can write this expression: x + ½y