Let v be the boats speed in still water.
Downstream speed = v + 8 km/hr
Time to get to 56 km: t = 56/(v + 8)
Upstream speed = v - 8 km/hr
Time to get to 24 km: t = 24/(v - 8)
The times are the same so: 56/(v + 8) = 24/(v - 8)
56(v - 8) = 24(v + 8)
32v = 80*8
v = 80*8/32
v = 80/4
v = 20 km/ hr