Hi everyone. I just have three questions that I can't answer and if anyone could spare the time to help me I will be very thankful :)

1.

A pile of sand is in a roughly conical pile. The slant edge of the pile is 6.0m.

a) what is the height of the pile when the volume of sand in it is at a maximum?

b) a truck operator contracted to shift the sand wants to know how many cubic metres there are. What answer should he be given? Justify your answer with regerence to the situation described.

2.

Nico's bubble blowing machine blows spherical bubbles whose surface area, S, increases at the rate of 23 cm²/s.

What is the rate of increase of the radius, r, of the bubble when the radius is 5 cm?

3.

The height in metres of a bullet fired up into the air after t seconds is given by h = 2000 t - 200 t²

Find the maximum height the bullet reaches. (A test to find the maximum height is required)