explain why XXXX , VX , IC , XVV are not used?
Here is the answer to your question.
The symbols I, X, C, M can be repeated 3 times at any given time. The symbols V, L, D can be used only once at any one instance.
Consider XXXX and XVV,
Here X is used 4 times where as V is used twice in XXXX and XVV respectively.
Thus, XXXX and XVV are not used.
The symbol I can be added to or subtracted from the symbol V and X only.
Consider VX and IC,
Here V and I are subtracted from X and C respectively. But V cannot be subtracted from X and I cannot be subtracted from C.
Thus, VX and IC are not used.