F5 Mathematics?

Let y km/h be the speed of the ship in current water.

Speed of the ship when traveling downstream = (y + 3) km/h
Speed of the ship when traveling upstream = (y - 3) km/h

Time taken for the ship to travel from town A to town B = 840 / (y + 3) h
Time taken for the ship to travel back to town A = 840 / (y - 3) h

Total time taken (in h) :
[840 / (y + 3)] + [840 / (y - 3)] = 37.5
[112 / (y + 3)] + [112 / (y - 3)] = 5
[112(y - 3) / (y + 3)(y - 3)] + [112(y + 3) / (y + 3)(y - 3)] = 5(y + 3)(y - 3) / (y + 3)(y - 3)
112(y - 3) + 112(y + 3) = 5(y + 3)(y - 3)
112y - 336 + 112y + 336 = 5y² - 45
5y² - 224y - 45 = 0
(y - 45)(5y + 1) = 0
y = 45 or y = -1/5 (rejected)

Time taken for the ship to travel from town A to town B
= 840/(45 + 3)
= 17.5 h

06:00 + 17:30
= 23:30
= 11:30 p.m.

The ship arrived at town B at 11:30 p.m.