jusjmkol740 wrote:

Q1. How many even 4-digit numbers can be formed, so that the numbers are divisible by 4 and no two digits are repeated?

a) 336 b) 784 c) 1120 d) 1804 e) 1936

Q2. How many 4-digit numbers can be formed, so that each contains exactly 3 distinct digits?

a) 1944 b) 3240 c) 3850 d) 3888 e) 4216

Note: Found these questions. But not the OAs. Can anyone help? Thanks in advance.

Note: the above questions are beyond the GMAT scope.How many even 4-digit numbers can be formed, so that the numbers are divisible by 4 and no two digits are repeated?A. 336

B. 784

C. 1120

D. 1804

E. 1936

I believe with "no two digits are repeated" the question means that all 4 digits are distinct.

Number is divisible by 4 if the last two digits form a number divisible by 4.

Therefore last two digits can be:

00;

04;

08;

12

16;

...

96.

Basically multiples of 4 in the range 0-96, inclusive. Multiples of 4 in a range 0-96, inclusive are \(\frac{last \ multiple \ in \ the \ range \ - \ first \ multiple \ in \ the \ range}{4}+1=25\) (for more on this issue:

http://gmatclub.com/forum/totally-basic ... ml#p730075).

But 3 numbers out of these 25 are not formed with distinct digits: 00, 44, and 88. Hence the numbers we are looking for can have only 22 endings.

If there is 0 in the ending (04, 08, 20, 40, 60, 80 - total 6 such numbers), then the first and second digit can take 8 and 7 choices each = 56 choices total. As there are 6 numbers with 0 in ending, hence total 6*56=336.

If there is no 0 in the ending (total 22 - 6 with zero = 16 such numbers), then the first digit can take 7 choices (10 - 2 digits in the ending and zero, so total 3 digits = 7, as 4-digit number cannot start with zero) and the second digit can take 7 choices too (10 digits - 3 digits we've already used) = 7*7=49 choices total. As there are 16 numbers without zero in ending, hence total 16*49=784.

TOTAL: \(336+784=1120\).

Answer: C.

How many 4-digit numbers can be formed, so that each contains exactly 3 distinct digits?A. 1944

B. 3240

C. 3850

D. 3888

E. 4216

As there should be 3 distinct digits in 4, the number will have 2 same digits and other 2 distinct - \(aabc\) (1123, 3477, ...)

\(C^3_{10}=120\) - # of ways to choose 3 distinct digits out of 10;

\(C^1_3=3\) - # of ways to choose which digit will be represented twice;

\(\frac{4!}{2!}=12\) - # of permutation of digits \(aabc\);

\(C^3_{10}*C^1_3*\frac{4!}{2!}=4320\).

Now, out of these 4320 numbers some will start with zero, but 4-digit number cannot start with zero as in this case it becomes 3 digit number. So how many out of these 4320 start with zero? As no digit has any preferences so equal numbers will start with each digit: 1/10 will start with 1, 1/10 with 2, 1/10 with 3 and so on. Thus 1/10=0.1 of 4320 will start with 0 and 0.9 of 4320 will start with digit other than zero.

TOTAL: \(4320*0.9=3888\).

Answer: D.

Hope it helps.

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