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15 Oct 2020, 16:10
Kudos
Bookmarks
Hi Riri1234,
The number of options for the 'middle' digit depend on three 'rules':
1) The digit cannot be 0 and all 3 digits must be unique.
2) The 'middle' digit has to be DIFFERENT from the 1st digit.
3) The 'middle' digit has to be DIFFERENT from the 3rd digit.
Regardless of what the 1st digit is, that will remove one option from the possibilities for the 'middle digit.' In that same way, regardless of what the 3rd digit is, that will also remove one option from the possibilities for the 'middle digit.' Thus, there will only be 7 possibilities for the middle digit.
GMAT assassins aren't born, they're made,
Rich
The number of options for the 'middle' digit depend on three 'rules':
1) The digit cannot be 0 and all 3 digits must be unique.
2) The 'middle' digit has to be DIFFERENT from the 1st digit.
3) The 'middle' digit has to be DIFFERENT from the 3rd digit.
Regardless of what the 1st digit is, that will remove one option from the possibilities for the 'middle digit.' In that same way, regardless of what the 3rd digit is, that will also remove one option from the possibilities for the 'middle digit.' Thus, there will only be 7 possibilities for the middle digit.
GMAT assassins aren't born, they're made,
Rich
27 Oct 2020, 03:41
Kudos
Bookmarks
I solved this Q using a different approach - I used the combinations formula. But my answer was wrong. The way I solved it is:
There is a 3 digit no and all are non zero digits. So there are 9 ways to select the digits -1-9. However, because we need the no to be >700, the first digit can only take 7,8,9.
Further, the 3rd digit has to be odd because we need an odd no. The 2nd digit can be anything - even or odd doesn’t matter.
3C1*9C1*5C1
Which gave me an answer of 135. Since this wasn’t in the answer options, I was sure I’d done something wrong but guessed 105 anyway since it was closer to my answer.
Bunuel, could u please help me understand where I’m going wrong here?
Posted from my mobile device
There is a 3 digit no and all are non zero digits. So there are 9 ways to select the digits -1-9. However, because we need the no to be >700, the first digit can only take 7,8,9.
Further, the 3rd digit has to be odd because we need an odd no. The 2nd digit can be anything - even or odd doesn’t matter.
3C1*9C1*5C1
Which gave me an answer of 135. Since this wasn’t in the answer options, I was sure I’d done something wrong but guessed 105 anyway since it was closer to my answer.
Bunuel, could u please help me understand where I’m going wrong here?
Posted from my mobile device
27 Oct 2020, 18:17
Kudos
Bookmarks
Hi kajaldaryani46,
The number of options for the 'middle' digit depends on three 'rules':
1) The digit cannot be 0 and all 3 digits must be unique.
2) The 'middle' digit has to be DIFFERENT from the 1st digit.
3) The 'middle' digit has to be DIFFERENT from the 3rd digit.
Regardless of what the 1st digit is, that will remove one option from the possibilities for the 'middle digit.' In that same way, regardless of what the 3rd digit is, that will also remove one option from the possibilities for the 'middle digit.' Thus, there will only be 7 possibilities for the middle digit (not 9). If you recalculate with this one change, you will get the correct answer.
GMAT assassins aren't born, they're made,
Rich
The number of options for the 'middle' digit depends on three 'rules':
1) The digit cannot be 0 and all 3 digits must be unique.
2) The 'middle' digit has to be DIFFERENT from the 1st digit.
3) The 'middle' digit has to be DIFFERENT from the 3rd digit.
Regardless of what the 1st digit is, that will remove one option from the possibilities for the 'middle digit.' In that same way, regardless of what the 3rd digit is, that will also remove one option from the possibilities for the 'middle digit.' Thus, there will only be 7 possibilities for the middle digit (not 9). If you recalculate with this one change, you will get the correct answer.
GMAT assassins aren't born, they're made,
Rich
