You simply calculate it like a cone, but the height of the cone is the height to the top of the FILLED part, not all the way.
Half-filled is not enough information . . . there can be "half filled" meaning half the height of the cone, but can also be "half filled" meaning half the volume of the cone.
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If 5 L is 1/3, 15 L is all of it.
You wll also need to know its radius as well as its volume
Throw them all into a suitably sized measuring vessel marked with volumes and measure the amount of water they displace.
It is not possible to calculate the area given only the volume.
You cannot calculate volume of surface area. If you meant 20m3, then the volume would be 20,000 litres.