Will the first 10 volumes of a 20-volume encyclopedia fit upright in the bookrack shown above? (1) x = 50 centimeters. Clearly insufficient.

(2) Twelve of the volumes have an average (arithmetic mean) thickness of 5 centimeters. You most certainly can not assume that thickness of

each of the 20 volumes is 5 centimeters, moreover you can not even assume that thickness of

each of theses 12 volumes is 5 centimeters. Yes, the total thickness of these 12 volumes is 12*5=60 centimeters, but individual thicknesses could be {1, 1, 1, ..., 1, 49} or {2, 2, 2, ..., 2, 38} ... Next, you don't know

which 12 volumes have an average thickness of 5 centimeters: first 12? last 12? first 6 and last 6? So, this statement is not sufficient.

(1)+(2) It's certainly possible that all 20 volumes to have thicknesses of 5 centimeters and in this case the first 10 volumes will fit in the bookrack of 50 centimeters

but it's also possible that the last 12 volumes to have an average thickness of 5 centimeters and the first 8 volumes have thicknesses of 100 meters each (why not?) and in this case the first 10 volumes will NOT fit in the bookrack of 50 centimeters. Not sufficient.

Answer: E.

Hope it's clear.

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