I suppose this should be the answer.
The negation of the statement:
Contains a 5 an a 6
is:
Thats includes following things:
1. Neither 5 nor 6
2. Contains only 5
3. Contains only 6
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1. Neither 5 nor 6 = 7 * 8^{7} = 14680064
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2. Contains only 5( but not 6 )
We use the methodology of contains atleast single 5.
So, Contains only 5 = 8 * 9^{7} - 7 * 8^{7 }= 23583688
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3. Contains only 6 (but not 5)
= 23583688
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4. Now, total possible numbers without any restrictions = 9 * 10^{7}
Thus, answer = statement(4) - [statement(1) + statement(2) + statement(3)] = 28152560