A good photographic print is obtained by an exposure of two seconds at a distance of `20` cm from the lamp. Calculate the time of exposure required to get an equally good result at a distance of `40` cm.

Explanation

To solve the problem of determining the time of exposure required to obtain a good photographic print at a distance of 40 cm, we can follow these steps:

### Step 1: Understand the relationship between intensity and distance
The intensity of light (I) from a point source is inversely proportional to the square of the distance (d) from the source. This can be expressed mathematically as:
\[ I \propto \frac{1}{d^2} \]

### Step 2: Determine the initial conditions
We are given that:
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