How long does it take the ferris wheel to make one full rotation?
h(t)=9sin(20(t-pi/2))+12 , h in metres and t in seconds

This is a circular ferris wheel, with diameter of 9 meters, and the axis
mounted 12 meters off the ground.  The equation is the height of the
a point that starts out level with the axis, at 't' seconds after the wheel
starts up. 

Now here's where things start to go awry:

The period of the revolution is the time it takes for the angle
inside the sine to change by 2pi .

20(t - pi/2) = 2pi

20t - 10pi = 2pi

20t = 12pi

t = 12pi/20 = 1.885 seconds

I can't help it.  All I did was the math, and that was bullet-proof. 
It was up to whoever wrote the question to guarantee that the
answer makes sense.

For one thing, I don't believe the 'pi/2' belongs inside the parentheses
with the 't'.  But even if we take the 'pi/2' out of the parentheses, that
still only makes the period 3.183 seconds, which is still an uncomfortable
number ... especially for the riders.