7. Using the digits $1,2,3,4$ only once to form a 4-digit number, how many of them are divisible by 11 ?
(a) 4 ;
(b) 5 ;
(c) 6 ;
(d) 7 ;
(e) 8 .
7 Ans: (e).
If the first digit is 1 , then the number is divisible by 11 implies that the 3rd digit is 4 and there are two ways to put 2 and 3 . The cases when the first digit are $2,3,4$ respectrively can be dealt with similarly. Hence the total number is $4 \times 2=8$.