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You need to know how to calculate the volume of a cylinder and how to calculate the volume of a cone. We know that the two things have the same height and they also have the same area for their base. 

Let's say that the area of the base is x.

The volume of the cylinder = x * 30 (area of the base multiplied with the height)

The volume of the cone = x/3 * 30 (1/3 multiplied with the area of the base and the height)

This means that the cone has one third of the volume of the cylinder's. Therefore if I pour water from a full cone, I can fill the cylinder only to the third of its volume, and in this case, height. So the answer is 10 cm.