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'Weekly Problem 52 - 2010' printed from http://nrich.maths.org/

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If we let the fifth number be $x$, then the sixth number is $6+x= 15$, so $x=9$.
 
The seventh number is the sum of the fifth and sixth numbers, $9+15=24$.
 
 

This problem is taken from the UKMT Mathematical Challenges.

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