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Explanation:

Let the speed of P be x and the distance between A and B be d, so the speed of Q will be 2x.

According to the question, (1 + t)v = 2vt = $\frac{d}{6}$ (Let t be the travel time of Q)

⇒ t = 1, and d = 12v

Hence, the time taken by P to reach to B = $\frac{d}{v}$ = 12 hours.

Hence, option (d).