About .0369 cubic yards.
20.5"x21"x4"=1722 cubic inches
1 Cubic yard of concrete is 27 cubic feet (9'x9'x9')
1 cubic foot of concrete is 1728 cubic inches (12"x12"x12")
1 cubic yard = 27 x 1728 = 46656 cubic inches
1722/46656 is approximately .0369
This would be a little less than two 80# bags of concrete mix
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 will need 6.17 cubic yards of cement.
8.64 cubic yards of cement.
You will need 27.6 cubic yards of cement.
0.86 cubic yards is needed.
1.85 cubic yards.
1.53 cubic yards.
3.95 cubic yards of cement are needed for a slab that size.
A 40x60x6in slab requires 44.44 cubic yards of cement.
A 16' x 8' by 4" slab requires 3.56 cubic yards of cement.
You'll need 1 cubic yard.
About 24 cubic yards of cement is required for a slab that size.
A 40'x40'x6" slab requires 29.63 cubic yards of cement.