She should ask her parents how it works before doingt it
To find the maximum velocity you have to differentiate the function respect to r.
v = k(R - r)r^2 = kRr^2 - krr^2 = kRr^2 - kr^3
k and R are constants.
=> v' = 2kRr - 3kr^2 = kr(2R - 3r)
maximum velocity => v' = 0 => kr(2R - 3r) = 0
r = 0 or 2R - 3r = 0 => r = 2R / 3
Answer: r = 2R / 3
<h2>
Hello!</h2>
The answer is:
C. 
<h2>
Why?</h2>
The density is the relation between the mass and the volume of an object. We are given four options to chose, and we can find the correct one by doing the following calculations:

So, the correct options is C. 
Have a nice day!
The toy rocket hits the ground when h(t) = 0.
So, h(t) = 0 = -9t^2 + 81.
Solving the equation,
-9t^2 = -81
Dividing by -9
t^2 = 9
Taking the square root.
t = 3
The answer is 3 seconds.