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Quantitative Aptitude ( CCS) Number System

The unit digit of the following expression

(1!)99 + (2!)98 + (3!)97 + (4!)96 + …… (99!)1

The unit digit of (1!)^99 is 1.The unit digit of (2!)^98 is 4.

The unit digit of (3!)^97 is 6.The unit digit of (3!)^97 is 6.

The unit digit of (4!)^96 is 0.Now since we know that all the factorial numbers starting with 5! has its unit digit 0.

So we need not to calculate it.Thus the required unit digit= unit digit of the sum of the unit digits.Thus the unit digit is 7

Hence, option 3 is the correct answer.

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