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There are 24 different four-digit integers than can be [#permalink]
 04 Nov 2012, 10:39
Question Stats:
69% (01:48) correct
30% (01:14) wrong based on 2 sessions
2345 2354 2435 +5432 There are 24 different four-digit integers than can be formed using the digits 2, 3, 4 and 5 exactly once in each integer. The addition problem above shows 4 such integers. What is the sum of all 24 such integers? A. 24,444 B. 28,000 C. 84,844 D. 90,024 E. 93,324 I'll give the explanation later
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Re: There are 24 different four-digit integers than can be [#permalink]
04 Nov 2012, 20:12
Maxswe wrote: 2345
2354
2435
+5432
There are 24 different four-digit integers than can be formed using the digits 2, 3, 4 and 5 exactly once in each integer. The addition problem above shows 4 such integers. What is the sum of all 24 such integers?
A. 24,444
B. 28,000
C. 84,844
D. 90,024
E. 93,324
I'll give the explanation later There are 4 integers and 4 places. Thus, any integer can be in one place for 6 (3*2*1) combinations. That means, there would be exactly 6 numbers in which 2 is at first place, 6 in which 2 is at second place , 6 in which 2 is at third place and 6 in which 2 is at 4th place. Same holds good for 3,4,5 also. Now sum of all numbers, is basically sum of their positional values. eg. 2435 = 2000 +400+30+5 Therefore sum of all such 24 integers = 6* (2000 +3000+4000+5000) + 6* (200 +300+400+500) + 6* (20 +30+40+50)+ 6* (2 +3+4+5) to avoid too much calculation - =1000X +100X+10X+X (where X =6*14 =84) now we know 1000X is 84000 , eliminate A,B 100 X is 8400, so total 92000 sth, eliminate C,D. Ans E it is.
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Re: There are 24 different four-digit integers than can be [#permalink]
04 Nov 2012, 21:05
Here's how I did it -
If you consider the unit digit, you can have 6 of the numbers ending in 5, 6 ending in 4, 6 ending in 3 and 6 ending in 2. The sum of these would be 84. So we know that the answer will have 4 as the last digit. Eliminate B.
If you consider the tens digit, you can have 6 of the numbers ending in 5, 6 ending in 4, 6 ending in 3 and 6 ending in 2. The sum of these would be 84 again and we have a carry over of 8 from the sum of unit digits . So the sum of tens digit is 84 + 8 = 92. This means that the answer will have 2 in the tens digit. Eliminate A and C.
If you consider the hundreds digit, you can have 6 of the numbers ending in 5, 6 ending in 4, 6 ending in 3 and 6 ending in 2. The sum of these would be 84 again and we have a carry over of 9 from the sum of tens digits . So the sum of hundreds digit is 84 + 9 = 93. This means that the answer will have 3 in the hundreds digit. Eliminate D. Answer is E.
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Re: There are 24 different four-digit integers than can be [#permalink]
05 Nov 2012, 03:42
My approach was different..
I tried to find if there is any pattern in the all 24 Nos.
For that I tried different numbers 1,2 and 3.
with this,you can create 3!=6 Nos without repitation.
if you see the numbers are
123
132
213
231
312
321
here if you see, at Units place, each number comes twice, for tenth place also each numbe comes twice and same for 100th place.
which is 3 diff Nos x 2 times each= total 6 diff numbers
So,I compare it with our number 2,3,4,5.
4 diff Nos x 6 timers each = 24 total diff numbers
So, For unit place
2*6=12
3*6=18
4*6=24
5*6=30
-----------
84
adding this for all the places, we get last three digit of sum as 324
which is in our answer choice E
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Re: There are 24 different four-digit integers than can be [#permalink]
05 Nov 2012, 09:18
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Nice!
Here is another way to solve it using this formula
If there are n distinct numbers that are used to make all possible n-digit numbers then the sum of all such numbers is
= (n-1)! *(sum of n digits)*(11....n times)
In our example:
n= 4
(n-1) ! = ( 4-1 ) ! = 3 ! = 6
sum of digits = 14 (5+4+3+2)
111..n times = here n = 4 >>1111
So 6*14* 1111 = 93,324
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Re: There are 24 different four-digit integers than can be [#permalink]
06 Nov 2012, 03:25
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Re: There are 24 different four-digit integers than can be [#permalink]
06 Nov 2012, 08:44
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Here is how I did it in 0:28 seconds. First consider the thousands digit, all the numbers are placed 6 times here.
So the sum of the thousands digits is 6*5 + 6*4 + 6*3 + 6*2 = 84 --> cancel A, B
The sum of the hundreds digits is also 84 --> 84000 + 8400 = 92,400 --> cancel C, D
Hence the answer is E.
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Re: There are 24 different four-digit integers than can be [#permalink]
06 Jan 2013, 00:52
Maxswe wrote: 2345
2354
2435
+5432
There are 24 different four-digit integers than can be formed using the digits 2, 3, 4 and 5 exactly once in each integer. The addition problem above shows 4 such integers. What is the sum of all 24 such integers?
A. 24,444
B. 28,000
C. 84,844
D. 90,024
E. 93,324
I'll give the explanation later We have 4 digits and 4 places without repetition. Each digit will occupy 1000, 100, 10 and units place 6 times. Sum = 6[2222+3333+4444+5555] = 93,324 Answer: E
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Re: There are 24 different four-digit integers than can be
[#permalink]
06 Jan 2013, 00:52
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