Correct Answer - Option 1 : 10.5 hours

**Given:**

Speed of boat is 20 km/h

Speed of the current is 10 km/h

**Concept:**

Speed = Distance/Time

**Calculation:**

Let us assume that distance between P and Q is D

⇒ 3.5 = D/(20 - 10) + D/(20 + 10)

⇒ 3.5 = D × (1/10 + 1/30)

⇒ D = 3.5 × (30/4) = 26.25 km

⇒ Thrice more distance = 105 km

⇒ The time taken to cover the distance in upstream = 105/(20 - 10) = 10.5 hours

**∴ The required result will be 10.5 hours.**