In x years time, Peter will be 23 years old. How old is he now?

In x years time, Peter will be 23 years old. How old is he now?

Let Peter's current age be a. In x years time means we add x to a, so we're given:
a + x = 23

We want to find a, s we subtract x from each side to get:
a + x - x = 23 - x

Cancel the x terms on the left side and we get:
a = 23 - x