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How many three letter computer passwords can be formed (no repetition allowed) with at least one symmetric letter?

990
2730
12870
1560000

Correct Answer: 12870

Explanation:

Total number of password using all alphabets -Total
number of password using no symmetric alphabets
= (26 x 25 x 24 ) - (15 x 14 x 13 )
= 12870

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