Instantaneous rate of change refers to the derivative.

h' = (-pi/120)*15sin(pi*t/120)
h' = (-pi/8)*sin(pi*t/120)
h'(20) = (-pi/8)*sin(pi/6)
h'(20) = (-pi/8) * 0.5
h'(20) = -0.196
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The instantaneous rate of change of h is dh(t)/dt.

First, lets analyze the situation.

h=15cos(pi*t/120) + 18 (click on this link to see more)

dh(t)/dt = -(pi/8) sin (pi*t/120) (you can see it in the lowest part of this link)

So, when t=20, replace the t in the dh(t)/dt with 20

dh(20)/dt = -0.196 (click on this link to see more)

*Hope this will answer your question
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