Reffering to movie clips help

I need help with altering values of movie clips (couldnt think of another way to put it)

say i need to change the _x of movie clip ##_mc

being variables

i would normally use:
tellTarget ("_root."+A+B+"_mc")
since the i cant just say _root.G5_mc._x++; cuz i dont know if its G5_mc or G4_mc (follow? im horrible at explaining things!)

is there any other way of doing this? or do i have to use tellTarget?

i tried:
“_root.”+A+B+"_mc._x" += 1;
but it didnt like that too much