I used s and c for the speeds of the man (relative to the water) and of the current respectively, so s + c and s - c are the man's downstream and upstream speeds in relation to the ground.

For a fixed distance, such as the distance between P and Q, covered at constant speed in relation to the ground, that speed is inversely proportional to the time taken to travel the distance (speed = distance/time), which implies that the ratio of the man's times for his journeys from Q to P and from P to Q is the inverse of the ratio of his speeds for those journeys.