When a boat moves upstream and then returns to catch a dropped float, the time taken to catch the float depends only on the distance between them when the boat turns back and the sum of their speeds, because both the float and the boat are carried by the same stream velocity. The boat travels upstream for a distance of 1.5 km at a relative speed of 20 minus 5, which is 15 km/h, meaning it takes 0.1 hours to turn around. The float also drifts downstream at 5 km/h during this time, so the total distance between them when the boat turns back is 1.5 km plus 0.5 km, resulting in 2 km. As the boat returns downstream at 25 km/h and the float continues at 5 km/h, their relative speed of approach is 30 km/h, and the time to cover the 2 km gap is 2 divided by 30 hours, which equals 6 minutes.