1.53 cubic yards.
It depends on how deep it is. If it is 6 inches deep, you will need 77 yards.
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.
If the slab is the standard 4" deep, you will need at least 8.89 cubic yards.
4.074 cubic yards for every foot the slab is thick.
You need to specify the slab's width and the length as well.
You will need 6.17 cubic yards of cement.
8.64 cubic yards of cement.
You will need 27.6 cubic yards of cement.
1.85 cubic yards.
0.86 cubic yards is needed.
A 40x60x6in slab requires 44.44 cubic yards of cement.
3.95 cubic yards of cement are needed for a slab that size.
A 16' x 8' by 4" slab requires 3.56 cubic yards of cement.
You'll need 1 cubic yard.
A 40'x40'x6" slab requires 29.63 cubic yards of cement.
About 24 cubic yards of cement is required for a slab that size.
40" x 40" x 3" requires 0.1 cubic yards of cement. If you mean 40 x 40 feet, you will need 14.81 cubic yards of cement.