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'Fibonacci Deduction' printed from https://nrich.maths.org/
_ , _ , _ , 6 , _ , 15
Let the fifth number be $x$:
_ , _ , _ , 6 , $x$ , 15
The sixth number is $6+x= 15$, so $x=9$.
_ , _ , _ , 6 , 9 , 15
The seventh number is the sum of the fifth and sixth numbers, $9+15=24$:
_ , _ , _ , 6 , 9 , 15 , 24