Sum of packing fractions of simple cubic, BCC and FCC lattice is  ‘x’ and no. of crystal systems with two bravais lattices are ‘y’. Then xyis

Simple cubic  = 0.52 BCC              = 0.68 FCC              = 0.74 Total             = 1.94 (x) tetragonal and monoclinic are two crystal systems which have 2 bravais lattices each ,hence y=2 therefore   xy=0.97