Let's consider the number of years ago when the father was 10 times older than his son.
Let x be the number of years ago.
The father's age x years ago would be 48 - x, and the son's age x years ago would be 21 - x.
According to the given condition, the father's age x years ago was 10 times the son's age x years ago:
48 - x = 10 * (21 - x)
Now, we can solve this equation to find the value of x:
48 - x = 210 - 10x
Combining like terms:
9x = 210 - 48
9x = 162
Dividing both sides by 9:
x = 18
Therefore, 18 years ago, the father was 10 times older than his son.