Answer: 1.1 m/s
Step-by-step explanation:
Speed [tex]s[/tex] is a relation between the traveled distance [tex]d[/tex] and time [tex]t[/tex]:
[tex]s=\frac{d}{t}[/tex]
In this sense, we are told the man traveled a distance [tex]d=1 km=1000 m[/tex] for a time [tex]t=15 min \frac{60 s}{1 min}=900 s[/tex] before realizing he lost his hat.
With this information we can calculate the speed of the river's current, since we have the bridge where the hat fell as a reference point:
[tex]s=\frac{1000 m}{900 s}[/tex]
[tex]s=1.1 m/s[/tex] This is the speed of the river's current