Joseph is 42 years old and his son is 12. How many years ago was Joseph six times as old as his son?

Explanation

1. 01

   Let it be $t$ years ago: $42 - t = 6(12 - t)$.

2. 02

   $42 - t = 72 - 6t$, so $5t = 30$.

3. 03

   $t = 6$. Check: 6 years ago they were 36 and 6.