The upper chamber of an hour-glass is a cone of radius 3 inches and height 10
inches and, if full, it requires exactly one hour to empty. Assuming that the
sand falls through the aperture at a constant rate, how fast is the level falling
when:
a) the depth of the sand is 6 inches?
b) 105/2
minutes have elapsed from the time when the hour-glass was full in
the upper chamber?