Explanation:

Let speed of boat in still water = x km/h
Speed of stream current = y km/h
24/(x - y) + 36/(x + y) = 6h ………. (i)
36/(x - y) + 24/(x + y) = 13/2 h …………… (ii)
In these type of Questions, make factor of 24 and 36and choose the common values which satisfy the avobe equations.
24 = 2 , 3 , 4 , 6 , 8 , [12]
36 = 3 , 4 , 9, [12]
Choose the common factor i.e Put this value in equation (i)
24/(x - y) + 36/12 = 6
24/(x - y) + 3 = 6
x – y = 8
x + y = 12
Thus x + y = 12
x = 10, y = 2
Speed of current, y = 2 km/h.