The height, $h$, in metres, above the ground of a rider on a Ferris wheel can be modelled by the equation $h = 10 \cos\left[\frac{\pi}{15}(t - 2.5)\right] + 12$ where $t$ is the time in seconds.
Sketch a graph to model the function.
During what time intervals, over the first two rotations of the Ferris wheel, is the rider at or below a height of 15 metres?