The last 4!

Find the last four digits of ( 1 ! + 2 ! + 3 ! + 4 ! + + 99 ! ) 19 ( 1!+2!+3!+4!+\dots+99! )^{19} .

5977 0313 0467 6437

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2 solutions

Astro Enthusiast
Sep 13, 2014

Factorial of any number n n where ( 20 n ) ( 20\leq n ) has zeroes as its last four digits. So, ( 1 ! + 2 ! + 3 ! + 4 ! + + 19 ! ) ( 1!+2!+3!+4!+\dots+19! ) mod 10000 10000 is 0313 0313 . So 31 3 19 313^{19} mod 10000 10000 is 5977 \boxed{5977} .

While writing a solution, do take the pain to make it complete. Some people here don't know to simplify the last step @ Prestosa

Krishna Ar - 6 years, 9 months ago

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Does anyone know to get 313 19 {313}^{19} by hand?

Kartik Sharma - 6 years, 8 months ago

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You must be knowing, dont you?

Krishna Ar - 6 years, 8 months ago

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@Krishna Ar Yeah yeah but who has the time and energy to do such calculations/

Kartik Sharma - 6 years, 8 months ago

One of them is me

Vighnesh Raut - 6 years, 6 months ago

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 \equiv 1 m o d 4 1 mod 4 . and then the entire thing m o d 25 mod 25 which is just the sum from 1 ! 1! to 9 ! 9! m o d 25 mod 25 (as all numbers from and after 10 ! 10! 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 \equiv 13 m o d 25 13 mod 25 . and then I found out the remainder when 13^19 is divided by 25 Which is 13.19^9 m o d 25 mod 25 \equiv 13.19.11^4 m o d 25 mod 25 \equiv 13.19.21^2 m o d 25 mod 25 \equiv -9.13.19 m o d 25 mod 25 \equiv 2 m o d 25 mod 25 . So there we go the last two digits are 1 m o d 4 1 mod 4 and 2 m o d 25 2 mod 25 . The only option matching are when the last two digits are 77 77 .

Don't go by my solution . I admit that even I hate this kind of exploitative solutions

Arghyadeep Chatterjee - 2 years, 6 months ago

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