The ferris wheel has a diameter of 40ft, axle 25ft above ground. Three seconds after starting, your seat is at its highest point. The wheel makes 3 rev/min.

I got the function, which is y(t)=25+20cos pi/10(t-3)

and its derivative y'(t)=-2sin pi/10(t-3)

what I don't understand is how do you calculate how fast/slow y(t) is increasing when t=15? And how do you determine the fastest y(t) changes, and the seat where it changes the fastest?