m o d 120 \bmod 120

1 ! + 2 ! + 3 ! + + 120 ! 1!+2!+3!+ \cdots + 120!

Find the remainder when the number above is divided by 120 120 .


The answer is 33.

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1 solution

Razing Thunder
Oct 2, 2020

Observe that 5 ! = 120 0 ( m o d 120 ) 5! = 120 \equiv 0 \pmod{120} and that n! has 5! as its factor for any n >= 5. Then we have

1 ! + 2 ! + 3 ! + + 120 ! 1 ! + 2 ! + 3 ! + 4 ! ( m o d 120 ) 33 ( m o d 120 ) . \begin{aligned} 1!+2!+3!+ \cdots + 120! & \equiv 1!+2!+3!+4! \pmod{120} \\ & \equiv 33 \pmod{120}. \end{aligned} Similar problem by brilliant staff

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