Tính: a, 923 + 887 - 923 - 687 b, 39 x 178 - 78 x 39 + 30^2 c, 3 x 4^2 ÷ [500 - (7 x 35 + 253)]

A, 923+887-923-687. B, 39.178-78.39+30^2 C, 3.4^2÷[500-(7.35+253)]