Palindrome

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indiantiger: Master | Next Rank: 500 Posts; Posts: 362; Joined: Fri Oct 02, 2009 4:18 amFollowed by: 1 member; Thanked: 26 times

by indiantiger » Mon May 31, 2010 9:52 am

SOURCE : MGMAT TEST

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 got this wrong on the test. Would like to see a faster way to do it?

"Single Malt is better than Blended"

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Source: Beat The GMAT — Problem Solving | Mon May 31, 2010 9:52 am

fibbonnaci: MBA Student; Posts: 403; Joined: Tue Dec 22, 2009 7:32 pmFollowed by: 22 members; Thanked: 98 times

by fibbonnaci » Mon May 31, 2010 10:58 am

A B C D these 4 letters represent the digits.

D can take up 5 values (1,3,5,7,9)
To fit the palindrome the first and last digit should be the same. so A can take up only 1 value (the value selected in D)

B can take up 10 values (0,1,2,3,4,5,6,7,8,9) and thus C can take up 1 value, the value selected in B.

so total is 5*10 => 50

Thus the number of 4 digit palindromes is 50

hope this helps!

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indiantiger: Master | Next Rank: 500 Posts; Posts: 362; Joined: Fri Oct 02, 2009 4:18 amFollowed by: 1 member; Thanked: 26 times

by indiantiger » Mon May 31, 2010 1:43 pm

Muchos gracias, I realized my mistake later on. I did not read the question properly and forgot the question is only asking for odd numbers.

"Single Malt is better than Blended"

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