The image obtained with a convex lens is erect and its length is `4` times the length of the object. If the focal length of lens is `20 cm`, calculate the object and image distances.

Explanation

To solve the problem, we need to find the object distance (U) and image distance (V) for a convex lens given that the image is erect and 4 times the length of the object, with a focal length (f) of 20 cm.

### Step-by-Step Solution:

1. **Understanding Magnification**:
The magnification (M) for a lens is given by the formula:
\[
M = \frac{h'}{h} = \frac{V}{U}
...