Question

Why Roman numeral DM is incorrect​

Answer 1

Only I, X, and C can be used for subtraction (V, L, and D cannot). Therefore, the following pairs of letter are invalid: VX, VL, VC, VD, VM, LC, LD, LM, DM. When subtracting, the value of the letter being subtracted from cannot be more than 10 times the value of letter being used for subtraction.