Final answer:
The son is currently 3 years old.
Step-by-step explanation:
To solve this problem, we can set up an equation:
Let x represent the son's age now.
10 years ago, the father was 10 times older than the son, so the father's age 10 years ago was 10x.
Since the father is now 40, his age 10 years ago would have been 40 - 10 = 30.
Setting up the equation,
10x = 30
Dividing both sides by 10, we get x = 3
Therefore, the son is currently 3 years old.