A boat goes 48 km upstream and 72 km downstream in 12 hours. It goes 72 km upstream and 48 km downstream in 13 hours.

Let u = 1/(upstream speed) and d = 1/(downstream speed).

48u + 72d = 12 and 72u + 48d = 13.

Adding: 120(u + d) = 25, so u + d = 5/24.
Subtracting: 24(u − d) = 1, so u − d = 1/24.

Then u = 1/8 and d = 1/12. Upstream speed = 8 km/h and downstream speed = 12 km/h.

Stream speed = (12 − 8)/2 = 2 km/h (boat speed in still water = 10 km/h).

Tip: when distances swap between two equations, add and subtract the equations.