Going upstream: 432 km / 6 hours = 72 km/h Going downstream: 384 km / 4 hours = 96 km/h Going upstream against the current means that the effective speed is: r_boat - r_current = 72 Meanwhile, going downstream with the current means; r_boat + r_current = 96 Adding both equations (to cancel out r_current) gives: 2r_boat = 72 + 96 r_boat = 84 km/h Substituting back into one of the original equations: r_current = 12 km/h.
