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What could be a part of the following set of numbers: $1201, 2121, 3142, 4253, 5342$

$a) 1317$

$b) 2315$

$c) 2573$

$d) 3456$

Differences between the numbers are $920, 1021, 1111, 1089.$ It's not making sense. Increasing then decresing. So difference of difference too won't make any difference.

Adding something to the square of something too is not leading me anywhere.

Taking ratios is not helping either.

If I go after the digit sum, I get: $4, 6, 10, 14, 14.$ Digit sum of options is- $12, 11, 17, 18.$ Again dead-end.

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up vote 3 down vote accepted

Notice that for each number listed, the sum of the first and last digit is the sum of the middle two digits. The only number in the options satisfying this property is $3456$. Notice the question is not concerned with the sequence of numbers, but the set of numbers.

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That's a very nice observation. Pretty Impressive! – Ramit Nov 6 '13 at 5:25

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