A convex lens A of focal length 20cm and a concave lens B of focal length 5cm are kept along the same axis with a distance d between them. I a parallel lbeam of leight falling on A leaved B as a parallel beam, then d is equal to `"____________"`cm.

Explanation

To solve the problem, we need to find the distance \( d \) between a convex lens \( A \) with a focal length of \( 20 \, \text{cm} \) and a concave lens \( B \) with a focal length of \( -5 \, \text{cm} \) such that a parallel beam of light entering lens \( A \) exits lens \( B \) as a parallel beam.

### Step-by-Step Solution:

1. **Identify the Focal Lengths**:
- The focal length of lens \( A \) (convex lens) is \( f_1 = 20 \, \text{cm} \).
- The focal length of lens \( B \) (concave lens) is \( f_2 = -5 \, \text{cm} \).

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