@lennyfan,
Okay, to elaborate the answer, now that the solution is out:
Pressure is force per unit area. However, pressure of a liquid on the bottom of its container can also be expressed as depth (height) of the liquid times the density of the liquid times the acceleration due to gravity.
Now the tank can handle a maximum of 185 kiloPascals or 185000 Pascals. The maximum pressure will occur at the bottom of the spherical tank, since that is where the water is deepest. So,
185000 Pascals / (1000 kg/m^3 * 9.8 m/s^2) = 18.877551 m as the height of the water. (Pressure divided by density times acceleration)
That means (since the radius of the tank is 12 meters, making the diameter 24m), that there will be 5.122449 meters at the top of the tank that is unfilled.
Now the volume of a sphere is V = (4/3)*pi*r^3 (r = radius). So the total volume of the tank is (4/3) * pi * (12)^3 = 7238.2295 m^3.
The top part of the tank that isn't filled is called a spherical cap. If you take the volume of the spherical cap and subtract it from the volume of the sphere, you get the volume of water in the tank.
A nice spherical cap calculator is at
http://www.1728.com/sphere.htm. (You can also look up all the formulas at Wikipedia.) We know the sphere radius (12) and the cap height (5.122449). Plugging these in, we find that the volume is 848.45 m^3. Subtracting this from the sphere volume, we get 6389.78 and rounding down as specified, we get the answer of 6389.