Final answer:
The value of x in the equation x - y = 30, when y = 15, is x = 45.
Step-by-step explanation:
To find the value of x in the equation x - y = 30, we substitute the value of y into the equation. Since y = 15, the equation becomes x - 15 = 30. To isolate x, we add 15 to both sides of the equation, resulting in x = 45.