How many yards of cement for a 55 x 15 x 6 slab?

Answer 1

It would be helpful to have units of measurement in the equation. However, pouring a slab I'll presume your measurements are feet x feet x inches. The volume of this slab is 55 x 15 x 0.5 (6 inches is 0.5 ft) = 412.5 cubic feet

1 yard of cement (more likely concrete as a cement slab will crumble under load)

is equal to 27 cu.ft. So, 412.5 cu.ft. needed / 27 cu. ft. / cu. yd. = 15.2777...cu. yards.

you will need to order 15.27777...cu. yards, as a yard isn't sold in this decimal you will need to order 15 1/2 yards of concrete to pour your slab.