An electric lamp

Let be the intensity when placed at a distance 80 cm and be the intensity when placed at a distance 20 cm apart from the screen.

Now,

According to the question, let the new distance between the lamp and the screen be x such that even after covering the lamp with a thin paper the intensities at the screen is balanced.

So,

⇒

⇒

Thus, the lamp has to be moved by 80 cm – 56 cm = 24 cm.