Let the speed of boat in still water be

km/hr.

Speed of water = 1.5 km/hr.

As we know that speed of boat downstream is (x+y) km/hr and speed of boat upstream is (x-y) km/hr.

{Here x means speed of boat in still water and y means speed of the stream}

Now,in this question;

Speed upstream = (x-1.5) km/h

And speed downstream = (x+1.5) km/hr

We know that;

Distance = Speed × Time

When boat goes upstream,

Distance = (x-1.5)(5.5)

⇒ 5.5x - 8.25

When boat goes downstream,

Distance = (x+1.5)(5)

⇒ 5x + 7.5

Now,while going upstream and downstream, distance is same.

∴ 5.5x - 8.25 = 5x + 7.5

∴ 5.5x - 5x = 7.5 + 8.25

∴ 0.5x = 15.75

∴ x = 31.5

Hence, Speed of the boat in still water is 31.5 km/hr.