A telephoto combination consists of convex lens of focal length 30cm and a concave lens of focal length 15cm, the separation between two lens is 27.5cm. Where should be the photographic plate placed in order to photograph an object 10m in front of the first lens?

Explanation

To solve the problem of where to place the photographic plate in a telephoto combination of a convex lens and a concave lens, we will follow these steps:

### Step 1: Understand the Configuration
We have a convex lens with a focal length \( f_1 = 30 \, \text{cm} \) and a concave lens with a focal length \( f_2 = -15 \, \text{cm} \). The separation between the two lenses is \( d = 27.5 \, \text{cm} \). The object is placed \( u = -1000 \, \text{cm} \) (since the object is in front of the lens, we take it as negative) from the convex lens.

### Step 2: Find the Image Formed by the Convex Lens
Using the lens formula:
\[
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