A palindrome is a number that reads the same forward and backward. For example, 2442 and 111 are palindromes. If 5-digit palindromes are formed using one or more of the digits 1, 2 and 3 how many such palindromes are possible?
1. 12
2. 15
3. 18
4. 24
5. 27
Palindrome - GMAT prep Exam pack 1
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Take the task of building palindromes and break it into stages.A palindrome is a number that reads the same forward and backward. For example. 2442 and 111 are palindromes. If 5-digit palindromes are formed using one or more of the digits, 1, 2, 3, how many such palindromes are possible?
A) 12
B) 15
C) 18
D) 24
E) 27
Stage 1: Select the ten-thousands digit
We can choose 1, 2, or 3
So, we can complete stage 1 in 3 ways
Stage 2: Select the thousands digit
We can choose 1, 2, or 3
So, we can complete stage 2 in 3 ways
Stage 3: Select the hundreds digit
We can choose 1, 2, or 3
So, we can complete stage 3 in 3 ways
IMPORTANT: At this point, the remaining digits are already locked in.
Stage 4: Select the tens digit
This digit must be the SAME as the thousands digit (which we already chose in stage 2)
So, we can complete this stage in 1 way.
Stage 5: Select the units digit
This digit must be the SAME as the ten-thousands digit (which we already chose in stage 1)
So, we can complete this stage in 1 way.
By the Fundamental Counting Principle (FCP), we can complete all 5 stages (and thus build a 5-digit palindrome) in (3)(3)(3)(1)(1) ways ([spoiler]= 27 ways[/spoiler])
Answer: E
--------------------------
Note: the FCP can be used to solve the majority of counting questions on the GMAT. For more information about the FCP, watch our free video: https://www.gmatprepnow.com/module/gmat-counting?id=775
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Brent
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Since this is a 5 digit palindrome, the middle number would act like a mirror.prata wrote:A palindrome is a number that reads the same forward and backward. For example, 2442 and 111 are palindromes. If 5-digit palindromes are formed using one or more of the digits 1, 2 and 3 how many such palindromes are possible?
1. 12
2. 15
3. 18
4. 24
5. 27
Case 1: Middle number = 1
_ _ 1 _ _
Number of options for 1st place = 3
Number of options for 2nd place = 3
Number of options for 4th and 5th place = 1
Total ways = 3*3 = 9
Case 2: Middle number = 2
_ _ 2 _ _
Similar to the above reasoning,
Total ways = 3*3 = 9
Case 3: Middle number = 3
_ _ 3 _ _
Similar to the above reasoning,
Total ways = 3*3 = 9
Total number of ways = 9 + 9 + 9 = 27
Correct Option: E
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Hi prata,
There's a full discussion of this question here:
https://www.beatthegmat.com/palindrome-5 ... 85219.html
GMAT assassins aren't born, they're made,
Rich
There's a full discussion of this question here:
https://www.beatthegmat.com/palindrome-5 ... 85219.html
GMAT assassins aren't born, they're made,
Rich