Explanation:

- Let speed of boat in still water = x km/hr, speed of stream = y km/hr.

- Upstream speed = x - y; Downstream speed = x + y.

- From question: 180 km upstream takes 18 hrs ? (x - y) = 180/18 = 10 km/hr.

- Time for 480 km upstream = 480/(x-y) = 480/10 = 48 hr.

- Let time downstream for 480 km = t hours.

- 48 - t = 40 ? t = 8 hr

- Downstream speed = 480/8 = 60 km/hr.

- So now, x + y = 60 and x - y = 10,

- Adding: 2x = 70 ? x = 35 km/hr.

?? Correct Answer: Option 2, 35 km/hr