  We find the bounds of the size of the union of n sets satisfying the
condition that the intersection of any k sets is empty. We show that any number
between the upper and lower bounds can be realised, and in the measure version,
the realisation can be the most efficient. The realisation also holds for the
counting version, but not always in the most efficient way. The elementary
result should be known but surprisingly, we cannot find any reference.
