Let the speed of boat in still water be U km/hr.

⇒ Upstream speed = (U – 4) km/hr

⇒ Downstream speed = (U + 4) km/hr

We know, Time = Distance / Speed

⇒ Time taken in upstream movement = 20/(U – 4)

 And, Time taken in downstream movement = 30/(U + 4)

The total time it takes is 7 hours and 30 minutes.

7 hours and 30 minutes is 7.5 hours.

⇒ 20/(U – 4) + 30/(U + 4) = 7.5

⇒ 20U + 80 + 30U – 120 = 7.5U² – 120

⇒ 15U² – 100U – 160 = 0

⇒ U = 8 or U = -4/3

Only possible option is 8.