let the speed of the boat in still water be x km/hr and speed of the current be y km/hr
speed of the boat in upstream = (x-y) km/hr
speed of the boat in down stream = (x+y) km/hr
since time = distance / speed
it goes 24km upstream and 28 km down stream in 6 hours.
it goes 30km upstream and 21km downstream in 6 1/2 hrs. i.e. 13/2 hrs therefore
eq (1) and eq(2) becomes:
multiplying eq(1)' by 3 and subtracting eq(2)' :
put u = 1/6 in eq(1)' ;
adding eq(3) and eq(4):
subtracting eq(3) from eq(4):
thus the speed of the boat in still water is 10km/hr and speed of the current is 4km/hr
hope this helps you.