Remainder from factorial series...

What is the remainder when

1 ! + 2 ! + 3 ! + + 50 ! 1! + 2!+3!+ \cdots +50!

is divided by 5 ! ? 5!\, ?

0 22 11 33

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6 solutions

All the terms after 4! i.e. 5!, 6!, 7!... can be written in terms of 5!(k), where k is some natural number...So the remainder will be formed by the terms in the beginning before 5!, i.e. 1!+2!+3!+4!=33

CHEERS!!:)

Cheers!! :)

math man - 6 years, 8 months ago

Brilliant, this one really stumped me

Daniel Habib - 4 years, 9 months ago

C h e e r s ! ! Cheers!!

Anik Mandal - 6 years, 8 months ago
Anna Anant
Jan 6, 2015

5!,6!,7! Is divisible by 5! Thus dividing 5!+6!+7!...+50! By 5! Will leave a zero remainder. 1!+2!+3!+4! Will be the residue. Therefore,1+2+6+24=33(remainder)

Rusli Azis
Apr 22, 2015

All the terms after 4! have a factor 5! So the remainder of 1!+2!+3!+4!+...+50! is equal to 1!+2!+3!+4! = 33 . In other words, we can simplify this terms into the form 1!+2!+3!+4!+5!(n),,so the remainder will be determined by the terms before 5!(n) .It obvious the remainder is 33

Prokash Shakkhar
Dec 7, 2016

We know 5 ! 5! is the multiple of ( 5 + k ) ! (5+k)! where, k N k \in N So, it can be easily said that 6 ! , 7 ! , 8 ! 6!, 7!, 8! \ldots \infty will be divided by 5 ! 5! ... So, ( 1 ! + 2 ! + 3 ! + + 50 ! ) ( 1 ! + 2 ! + 3 ! + 4 ! ) ( m o d 5 ! ) (1! + 2! + 3!+ \ldots + 50!)\equiv (1! +2!+3!+4!)\pmod{5!} ...... So, the remainder is 1 ! + 2 ! + 3 ! + 4 ! = 33 1!+2!+3!+4! = \boxed{33}

Priyanshu Mishra
Dec 28, 2014

We observe that in the series, after 4! the remainder is 0 in each case when divided with 5!.
Now we are left with 1! + 2! + 3! + 4!. as 1! + 2! + 3! + 4! = 1 + 2 + 6 + 24 = 33, the remainder will be 33. also , 33 is congruent to 33 (mod 5!).

Chenjia Lin
Jan 31, 2016

Notice that the terms from 5 ! 5! to 50 ! 50! all contain a 5 ! 5! in their product, which means that their units digit are either 0 or 5. Theoretically speaking, if you add all the units digits of 5 ! 5! to 50 ! 50! , the sum will be divisive by 5 ! 5! . This means that we only need to calculate the sum from 1 ! 1! to 4 ! 4! to see if it's divisible by 5. Adding the first four terms, we get 33 \boxed{33} .

Moderator note:

Your solution finds the remainder when divided by 10, and not when divided by 5!. How can we approach this problem if it wasn't multiple-choice?

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