The quantities in the proportion you give add to 16 pounds, of which 2 pounds are cement. That is, cement constitutes 1/8 of the total weight.
For each yard of concrete, 4000 lb/8 = 500 lb of cement are used.
For 12.5 yards of concrete, 12.5 * 500 = 6,250 pounds of cement will be used.
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"yard" in this case is shorthand for "cubic yard".