Terminate quickly

0 ! + 1 ! + 2 ! + 3 ! + + 2013 ! + 2014 ! \large 0! + 1! + 2! + 3! + \ldots + 2013! + 2014!

What are the last two nonzero digits of the number above?


The answer is 14.

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2 solutions

For n 10 n \ge 10 we have that n ! n! has the factors 2 , 5 2,5 and 10 , 10, and thus its last two digits will both be 0. 0. Thus we just need to determine

( n = 0 9 n ! ) ( m o d 100 ) = n = 0 9 ( n ! ( m o d 100 ) ) \left(\displaystyle\sum_{n=0}^{9} n!\right) \pmod{100} = \sum_{n=0}^{9} (n! \pmod{100})

( 1 + 1 + 2 + 6 + 24 + 20 + 20 + 40 + 20 + 80 ) ( m o d 100 ) \equiv (1 + 1 + 2 + 6 + 24 + 20 + 20 + 40 + 20 + 80) \pmod{100}

214 ( m o d 100 ) = 14 . \equiv 214 \pmod{100} = \boxed{14}.

Moderator note:

Splendid. By knowing its trailing zeros, one could easily "terminate" the irrelevant terms to find the last few digits of 1 ! + 2 ! + + 2014 ! 1! + 2!+\ldots+2014! .

Nicely done sir!Did it the same way as yours.

Arian Tashakkor - 6 years, 1 month ago

The Factorial of all the numbers starting from 10 has the last two digits (atleast) as 0 and 0. This means we have to find the last two digits of

n = 0 9 \displaystyle \sum_{n=0}^9

which is

n = 0 9 ( m o d 100 ) 1 + 1 + 2 + 6 + 24 + 20 + 20 + 40 + 20 + 80 ( m o d 100 ) 214 ( m o d 100 ) = 14 \displaystyle \sum_{n=0}^9 \pmod {100} \equiv 1 + 1 + 2 + 6 + 24 + 20 + 20 + 40 + 20 + 80 \pmod {100} \equiv 214 \pmod {100} = 14

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