The frequency (f) of oscillation is the number of oscillations (n) per time (t) in seconds: f = n/t. A duck floating on a lake oscillates up and down 5.0 times (n = 5.0) during a 10.-second interval (t = 10.0 s). So, the frequency of duck's oscillations is: f = 5.0/10.0 s = 0.50 1/s = 0.50 Hz. :)