What is a half multiplied by a third?
1/2 x 1/3 = 1/6
The formula for finding the volume of a cylinder is pi multiplied by radius squared multiplied by the height. So for this case it would be pi multiplied by 6 squared multiplied by 7.5. But since you want it when it is HALF full, you would divide by two, so the answer would be 135 multiplied by pi, or around 424 metres squared of water.
There are several ways for such a number to be constructed: It can be the seventh power of a prime. It can be the third power of a prime multiplied by the third power of another prime. It can be the third power of a prime multiplied by the first power of two other primes. It can be the first power of three different primes.