# permutation and combination

This topic contains 10 replies, has 4 voices, and was last updated by  shruti290 6 years, 1 month ago.

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• #499

shruti290
Participant

q1.)the number of numbers divisible by 3 tht can be formed by four different even digits is?
q2.)the no of 4 digit no tht can be made with the digits 1,2,3,4 and 5 in which at least 2 digits are identical?

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• #501

rohit pandey
Participant

for q2
at least 2 digits same means
we take cases for
1.2 same digits
2.3 same digits
3.4 same digits

for case 1-5*4*3*2/2!
for case 2 – 5*4*3*2/3!
for case 3 – 5*4*3*2/4!
now add all of them up
this is a probable solution

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• #503

abhisheksharma12593
Participant

atleast 2 are identical means either 2 are identical or 3 are identical or 4 are identical
no of ways if 2 are identical are 5*4*3*2/2!
no of ways if 3 are identical are 5*4*3/3!
no of ways if 4 are identical are 5*4*3*2/4!
adding all three we get 60+20+5=85 ways

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• #506

abhisheksharma12593
Participant

atleast 2 are identical means either 2 are identical or 3 are identical or 4 are identical
no of ways if 2 are identical are 5*4*3*2/2!
no of ways if 3 are identical are 5*4*3*2/3!
no of ways if 4 are identical are 5*4*3*2/4!
adding all three we get 60+20+5=85 ways
what you are saying is that either 1,2 or 2,3 or 3,4 are identical…plz explain this

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• #502

rohit pandey
Participant

for q 1

even digits are 0 , 2 , 4 ,6 ,8

so probable 4 digits are 1.0246=0+2+4+6=12
2.2468=2+4+6+8=20
3.4680=4+6+8+0=18
4.6802=6+8+0+2=16
5.8024=8+0+2+4=14

only option 1 and option 3 is divisible by 3

so permutation of option1=4!-3!
( we do -3! factorial because we need 4 digit numbers and thus we fix 0 at fourth place and form permutations and subtract it from total permutations}
and
permutation of option2=4!-3! {same thing again}

where did you get this question?

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• #509

shruti290
Participant

i did not follow what u said and the correct ans is 505 i forgot to mention and i got it
basically the question says tht atleast 2 didigts are identical which is just another way of saying tht at least one digit is repeated therefore the answe is
5^4-5!
that is the total no of ways- no digit repeated

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• #513

abhisheksharma12593
Participant

i didn’t get this at all…….let admin come to rescue!!!

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• #510

shruti290
Participant

the correct answer is 36 and yours is 18

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• #518

shruti290
Participant

Why did u take o ?
And tell me one thing it says “the no of numbers” so shouldn’t we take the one digit two digit three and four digit no’s As well and since it says diff even do digits therefore we cannot exceed beyond four digits ?

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• #526

Keymaster

For q1, Rohit is right.. and Shruti, his answer is 36 indeed (not 18)

For q2, Shruti is right
#ways at least 2 identical = total ways – #ways none identical
…this is the same approach of using the complement, which is often easier

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• #535

shruti290
Participant

but sir why did he take 0?

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