A convex lens is coaxially placed with a concave lens. A screen is placed at a distance of 100 cm apart from the combination. The focal length of convex lens is 15 cm. Determine the focal length of the concave lens if image of object placed at a distance of 20 cm, from the combination, is projected on the scrcen.

Explanation

To solve the problem, we will follow these steps:

### Step 1: Identify the given values
- Focal length of the convex lens (f1) = +15 cm (positive because it's a convex lens)
- Object distance (u) = -20 cm (negative because the object is placed on the same side as the incoming light)
- Image distance (v) = +100 cm (positive because the image is formed on the opposite side of the lens)

### Step 2: Use the lens formula for the combination of lenses
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