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Explanation:

As the person covers 4.8 km, he covers 2.4 kms on one side and 2.4 kms on other side.

Let the number of stones be ‘n’ on either side of the road excluding the middle stone.

So total distance covered by him on one side = 20 + 40 + 60 + … + 20n

∴ 2400 = n2 (2 × 20 + (n - 1)20) = 10n (n + 1)

(Here n is the number of stones)

After solving, we get n = 15

∴ Total number of stones = 15 + 15 + 1 = 31

Hence, option (d).

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