Find the last four digits of ( 1 ! + 2 ! + 3 ! + 4 ! + ⋯ + 9 9 ! ) 1 9 .
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While writing a solution, do take the pain to make it complete. Some people here don't know to simplify the last step @ Prestosa
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Does anyone know to get 3 1 3 1 9 by hand?
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You must be knowing, dont you?
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@Krishna Ar – Yeah yeah but who has the time and energy to do such calculations/
One of them is me
I know it's a sin in my view but I did the problem by eliminating the options . I first found out the number mod 10 to eliminate the 3rd option . Then I just found out the last two digits. First I calculated the remainder when the entire thing is divided by 4 which is just 9^19 mod 4 ≡ 1 m o d 4 . and then the entire thing m o d 2 5 which is just the sum from 1 ! to 9 ! m o d 2 5 (as all numbers from and after 1 0 ! is divisible by 25 . I wont lie, I used a calculator in this step to add the factorials from 1 to 9. which came out to be 409113 mod 25 which was obviously ≡ 1 3 m o d 2 5 . and then I found out the remainder when 13^19 is divided by 25 Which is 13.19^9 m o d 2 5 ≡ 13.19.11^4 m o d 2 5 ≡ 13.19.21^2 m o d 2 5 ≡ -9.13.19 m o d 2 5 ≡ 2 m o d 2 5 . So there we go the last two digits are 1 m o d 4 and 2 m o d 2 5 . The only option matching are when the last two digits are 7 7 .
Don't go by my solution . I admit that even I hate this kind of exploitative solutions
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Factorial of any number n where ( 2 0 ≤ n ) has zeroes as its last four digits. So, ( 1 ! + 2 ! + 3 ! + 4 ! + ⋯ + 1 9 ! ) mod 1 0 0 0 0 is 0 3 1 3 . So 3 1 3 1 9 mod 1 0 0 0 0 is 5 9 7 7 .