A right cylindrical tank standing on end holds 120 gallons of water
when filled to capacity, and is 24 inches high. The tank is filled to
the 21 inch mark, and 60 gallons then drained out.
Find the height of the remaining water, i.e., find x.
If 60 gallons were drained from a full 120 gallon tank, the height
of the remaining waer would be at the half-full level of 12 inches.
So, irrespective of the height of the water before draining, draining
60 gallons lowers the height of the water 12 inches.
So, if the tank is filled to the 21 inch mark, draining 60 gallons would
lower the height of the watrer to 21 minus 12 inches = 9 inches.