Solve and show how the porosity of this packing give this value and explain

Factors affecting porosity-Effect of Packing Packing of identical sphere leads to a porosity that is grain size independent. Suppose a rectangular reservoir is 10 miles long, 3km wide and 120 feet thick; and suppose the porosity of a rock sample obtained from the reservoir is 25%. The bulk volume of the reservoir is then. Assume the sample is representative of the reservoir, i.e., the reservoir rock is the same everywhere