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Find the last two digits of the following sum: 5! + 6! + 7! + 8! + 9! + 10! + ... + 100!

 Feb 14, 2021
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Any factorial greater than 10! will end in 00. This means we just need to find what 5! + 6! + 7! + 8! + 9! ends in. 

 

5! = 120

6! = 720

7! = 5040

8! = 40320

9! = 362880

 

Add up the last two digits of these numbers and you get 180, which means the last two digits of the sum is 80, which is your answer :)

 Feb 14, 2021

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