1) If you want to start at zero i.e. 0-9, 10-19 etc, enter a formula like:
 
=FLOOR('Number Groups'[Num],10)&" - "&CEILING('Number Groups'[Num],10)+IF(MOD('Number Groups'[Num],10)>0,-1,10-1)
 
2) If you want to start at one i.e. 1-10, 11-20 etc, enter a formula like:
 
=FLOOR('Number Groups'[Num],10)+IF(MOD('Number Groups'[Num],10)>0,1,-(10-1))&" - "&CEILING('Number Groups'[Num],10)
 
3) 2) If you want to start at ten i.e. <10, 10-19 etc, enter a formula like:
 
=IF('Number Groups'[Num]<10,"<"&10,FLOOR('Number Groups'[Num],10)&" - "&CEILING('Number Groups'[Num],10)+IF(MOD('Number Groups'[Num],10)>0,-1,10-1))