Thirty is half of the sum of 4 and a number

Thirty is half of the sum of 4 and a number.

The phrase a number represents an arbitrary variable, let's call it x.

The sum of 4 and a number:
4 + x

Half of this sum means we divide by 2:
(4 + x)/2

Set this equal to 30:
(4 + x)/2 = 30 <-- This is our algebraic expression