Discussion

Find the missing number :

Answer: C

The missing number is 8.

In each row, the middle number is the sum of the squares of the outer two:

\[ \begin{aligned} 5^{2} + 9^{2} &= 25 + 81 = 106 \\ 6^{2} + 7^{2} &= 36 + 49 = 85 \\ 4^{2} + ?^{2} &= 80 \Rightarrow ?^{2} = 64 \end{aligned} \]

So the missing number is 8. 11 and 15 would make the middle number 137 and 241, and 3 would make it 25.

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