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student4gmat
- Junior | Next Rank: 30 Posts
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- Joined: Sat Jun 14, 2014 8:45 am
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Using digits 1 - 9, in how many ways can you form a 4 digit no. which is odd and has the digit 9 exactly once. (without repetition)
My method-
1st case: when 9 comes in the unit digit then the no. of ways will be : 8 * 7 * 6 * 1 = 336
2nd case: when 9 does not come in the unit's digit then we have 4 options to fill units place and at thousands place we can have 9 hence the no. of ways will be : 1 * 7 * 6 * 4 = 168
However we can have 9 at the hundreds place or tens place as well so the send case 1 * 7 * 6 can be arranged in 3! ways so 168 * 6 = 1008
answer: 1008+336 = 1344
This is not the correct answer....can someone please help where I am going wrong. Thanks.
My method-
1st case: when 9 comes in the unit digit then the no. of ways will be : 8 * 7 * 6 * 1 = 336
2nd case: when 9 does not come in the unit's digit then we have 4 options to fill units place and at thousands place we can have 9 hence the no. of ways will be : 1 * 7 * 6 * 4 = 168
However we can have 9 at the hundreds place or tens place as well so the send case 1 * 7 * 6 can be arranged in 3! ways so 168 * 6 = 1008
answer: 1008+336 = 1344
This is not the correct answer....can someone please help where I am going wrong. Thanks.
