Discussion

Explanation:

Simplify:

9 - [10 - {20 - (15 - $\overline{9-3}$)}].

= 9 - [10 - {20 - (15 - 6)}]

= 9 - [10 - {20 - 9}]

= 9 - [10 - 11]

= 9 - [-1]

= 10

Hence, option (a).

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