Question 713125
The tank volume is radius squared times pi, times height, divided by 3.
In cubic meters, that is 3 squared, times pi, times 9, divided by 3,
which calculates to 27pi
The water in the tank occupies a similar cone, 
with all dimensions proportionally smaller by a factor of 3.
That volume is smaller by a factor of 3 cubed, or 27.
So the volume of water in the tank is pi cubic meters.
The volume when the water is twice as deep is the volume of a similar cone,
2 times as large in all dimensions.
That makes the volume larger by a factor of 2 cubed, or 8.
The amount of water that needs to be added is 7 times what is in the tank,
or 7pi cubic meters.
That is about 22 cubic meters, which is 22,000 liters.