[quote="funkyakki"][quote="GMATinsight"][quote="MBADream786"]Find the sum of all the numbers that can be formed by taking all the digits at a time from 3, 4, 6, 7 and 9 without repetition?
A.7733652
B.7733256
C.7373256
D.7373652
E.7733653[/quote]
Total Numbers that can be formed using 3, 4, 6, 7 and 9 without repetition = 5! = 120
Every digit (5 digits) has equal chance to appear at unit place i.e. each digit will appear at unit digit = 120/5 = 24 times
i.e. Sum of unit digits of 120 numbers = 24*3+24*4+24*6+24*7+ 24*9 = 24*(3+4+6+7+9) = 696
i.e. sum of all numbers = 696*10000+696+1000+696*100+696*10+696 = 7733256
Answer: Option B[/quote]
Can you please explain this part
[highlight]Sum of unit digits of 120 numbers = 24*3+24*4+24*6+24*7+ 24*9 = 24*(3+4+6+7+9) = 696[/highlight][/quote]
We have 5 no. With us to form 5 digit no. Without repetion.
Let's say. Unit digit is 9
Then other 4 no. (Tens, hundreds, thousands, ten thousands) can be written as 4! = 24
So there will be 24 no. With 9 as unit digit..
Similar with others.
So, the summation will be
9*24 + 7*24 + 6*24 + 4*24 + 3*24 = 696
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