Sean is 20 and his brother is 12. How many years ago was Sean three times as old as his brother?

MY WORK:

20 - x = 3(x)

20 - x = 3x

20 = x + 3x

20 = 4x

20/4 = x

5 = x

Sean's current age is 20.

Sean's age some years ago is 20 - x.

I just found x to be 5.

So, 20 - 5 = 15.

15 is three times more than 5.

So, when Sean was 15 years old, his brother was 5.

Is this correct?