Explanation:

Let's break down the problem:

- The shopkeeper buys millet at Rs 16 per kg.

- He sells it at Rs 20 per kg.

- Instead of 1000 gm, he gives only 800 gm.

To find the actual profit percentage:

- Cost for the shopkeeper for 800 gm: $\frac{16}{1000}\times 800=Rs12.8$.

- The selling price for 800 gm is Rs 20.

Profit is: $Rs20-Rs12.8=Rs7.2$.

- Profit percentage: $\frac{7.2}{12.8}\times 100=56.25\mathrm{\%}$.

Therefore:

- Option 4: 56.25%

This is the correct answer.

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