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Fibonacci Deduction

Stage: 3 and 4 Short Challenge Level: Challenge Level:1

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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