A palindrome is a number that reads the same forward and backward, such as 121. How many odd, 4-digit numbers are palindromes?
A. 40
B. 45
C. 50
D. 90
E. 2500
[spoiler]OA = C [/spoiler]
I tried to search this topic but couldnt find it. The source is MGMAT, I couldnt get their explanation. Please give your inputs with detailed explanations.
Thanks in advance.
A palindrome
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odds end in 1, 3, 5, 7, and 9 - call the set of these numbers x
so we have
x _ _ x
the two blanks must have the same number from 0-9 -- so 10 combinations for each blank
5 numbers in x * 10 combinations = 50 = C
so we have
x _ _ x
the two blanks must have the same number from 0-9 -- so 10 combinations for each blank
5 numbers in x * 10 combinations = 50 = C
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Take the task of building palindromes and break it into stages.
Begin with the most restrictive stage.
Stage 1: Select the units digit
We can choose 1, 3, 5, 7 or 9
So, we can complete stage 1 in 5 ways
Stage 2: Select the tens digit
We can choose 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9
So, we can complete stage 2 in 10 ways
IMPORTANT: At this point, the remaining digits are already locked in.
Stage 4: Select the hundred digit
This digit must be the SAME as the tens digit (which we already chose in stage 2)
So, we can complete this stage in 1 way.
Stage 5: Select the thousands digit
This digit must be the SAME as the units 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 4 stages (and thus build a 4-digit palindrome) in (5)(10)(1)(1) ways ([spoiler]= 50 ways[/spoiler])
Answer: C
--------------------------
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So we have the 4-digit numbers in the form of ABBA where A is an odd number and B can be any digit including B = A.tata wrote: ↑Mon Mar 01, 2010 10:30 amA palindrome is a number that reads the same forward and backward, such as 121. How many odd, 4-digit numbers are palindromes?
A. 40
B. 45
C. 50
D. 90
E. 2500
[spoiler]OA = C [/spoiler]
I tried to search this topic but couldnt find it. The source is MGMAT, I couldnt get their explanation. Please give your inputs with detailed explanations.
Thanks in advance.
Therefore, we have 5 choices for the first A and 10 choices for the first B. However, since the second A and B must be the same as the first A and B, respectively, there is only 1 choice for each of the second A and B. So we have 5 x 10 x 1 x 1 = 50 such numbers.
Answer: C
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