A father tells his son,"I was of your present age when you were born".If the father is 46 now.How old was the boy 5 years back?

Explanation

To solve the problem step by step, let's break it down clearly:

1. **Understanding the Statement**: The father states, "I was of your present age when you were born." This implies that the father's current age is equal to the sum of his age at the time of the son's birth and the son's current age.

2. **Assign Variables**: Let the current age of the son be \( x \) years. According to the problem, the father's current age is given as 46 years.

3. **Setting Up the Equation**: Since the father was \( x \) years old when the son was born, the father's age at that time plus the son's current age equals the father's current age. This can be expressed as:
\[ \text{Father's age when son was born} + \text{Son's current age} = \text{Father's current age} \]
This translates to:
\[ x + x = 46 \]
or
\[ 2x = 46 \]

4. **Solving for \( x \)**: To find the son's current age, we solve the equation:
\[ x = \frac{46}{2} = 23 \]
So, the son is currently 23 years old.

5. **Finding the Age 5 Years Ago**: To find the son's age 5 years ago, we subtract 5 from his current age:
\[ \text{Son's age 5 years ago} = 23 - 5 = 18 \]

6. **Conclusion**: Therefore, the son was 18 years old 5 years back.

### Final Answer:
The boy was 18 years old 5 years back.