A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different positions separated by 20 cm. Calculate the focal length of the lens.

Explanation

To solve the problem, we will use the lens displacement formula related to the focal length of a convex lens. The steps are as follows:

### Step 1: Understand the given data
- The distance between the object and the screen (D) = 90 cm
- The separation between the two positions of the lens (d) = 20 cm

### Step 2: Use the lens displacement formula
The formula relating the focal length (f) of the lens to the distances is given by:
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