With 3 at unit place, total possible four digit number (without repetition) will be 3!=6 With 4 at unit place, total possible four digit numbers will be 3!=6 With 5 at unit place, total possible four digit numbers will be 3!=6 With 6 at unit place, total possible four digit numbers will be 3!=6 Sum of unit digits of all possible numbers = 6 x 3 + 6 x 4 + 6 x 5 + 6 x 6 =6[3+4+5+6] =6[18]=108