7. A laminated strip is built up by randomly selecting two layers of material A, three layers of material B, and four layers of material C. The thicknesses of the individual layers of material A have mean 0.0100 in., and standard deviation 0.0005 in.; the respective numbers for material B are 0.0050 and 0.0003 in., and those for C are 0.0025 and 0.0001 in. A large lot of such laminated strips are manufactured. Find the mean and standard deviation of the thicknesses of the strips in this lot.