2002 AMC 12A Problem 9

The files need $3(0.8)+12(0.7)$ $+15(0.4) = 16.8$ mb, so at least $\dfrac{16.8}{1.44} = 11\tfrac{2}{3}$ disks by volume alone. A disk containing a $0.8$-mb file has room for only one more $0.4$-mb file, leaving at least $0.24$ mb unused. Across the three $0.8$-mb files this wastes at least $3(0.24) = 0.72$ mb, over half a disk, forcing at least $13$ disks. Thirteen suffice: six disks each hold two $0.7$-mb files, three disks each hold one $0.8$-mb file plus one $0.4$-mb file, and four disks each hold three $0.4$-mb files. Thus, the correct answer is B.