At present a father's age is 30 years more than that of his only son. What was his age when the son was born?

Options

A. 40 years
B. 30 years
C. 35 years
D. 25 years

Explanation

To solve the problem step by step, we can follow these instructions:

1. **Define Variables**:
- Let the present age of the son be \( X \).
- According to the problem, the present age of the father is \( X + 30 \) (since the father is 30 years older than the son).

2. **Determine the Father's Age When the Son Was Born**:
- When the son was born, the father’s age would have been his current age minus the son’s current age.
- Therefore, the father's age when the son was born can be calculated as:
\[ \text{Father's age when son was born} = (X + 30) - X \]

3. **Simplify the Expression**:
- Simplifying the above expression:
\[ \text{Father's age when son was born} = 30 \]

4. **Conclusion**:
- Thus, the father was 30 years old when his son was born.