The boat travels the distance between two points on the river downstream in 600 seconds, up in 900 seconds.

The boat travels the distance between two points on the river downstream in 600 seconds, up in 900 seconds. How long does it take for a boat to travel this distance in still water?

The boat travels the same distance and the ratio is true:
(V – V1) * 900 = (V + V1) * 600 = V * t, where V is the speed of the boat, V1 is the current speed, 900 s and 600 s is the time of movement upstream and downstream, respectively, t is the time of movement in stagnant water.
(V – V1) * 900 = (V + V1) * 600.
1.5 (V – V1) = V + V1.
1.5V – 1.5V1 = V + V1.
0.5V = 2.5V1.
V1 = 0.2V.
(V – V1) * 900 = (V + V1) * 600 = V * t.
t = (V – 0.2V) * 900 / V = (V + 0.2V) * 600 / V = 0.8 * 900 = 1.2 * 600 = 720 s.
Answer: 720 p.