Cut Out Anything Unnecessary

Find the remainder when 1 ! + 2 ! + 3 ! + + 100 ! 1!+2!+3!+\ldots +100! is divided by 24.


The answer is 9.

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5 solutions

Mehul Chaturvedi
Jan 13, 2015

Upvote solution if you like it


As we know

4 ! = 24 , 5 ! = 24 × 5 , 6 ! = 24 × 6 × 5.......... \large 4!=24 , 5!=24\times5,6!=24\times6\times5 ..........

Means every number starting from 4 ! 4! will give us remainder as 0 0 on dividing by 24 24

Means we will get remainder from 1 ! + 2 ! + 3 ! = 9 \large 1!+2!+3!=9

and 24 m o d 9 = 9 \large 24 \mod 9=9

\therefore the answer is 9 \Huge\Rightarrow\color{royalblue}{\boxed{9}}

Paola Ramírez
Jan 14, 2015

4 ! 4! is 24 24 , then, for n ! 4 , 24 n ! n! \geq 4, 24|n! that means its remainder is 0 0

So

( 1 ! + 2 ! + 3 ! + + 100 ! ) ( 1 ! + 2 ! + 3 ! + 0 + . . . + 0 ) m o d 24 (1!+2!+3!+……+100!)\equiv(1!+2!+3!+0+...+0) \mod24

( 1 ! + 2 ! + 3 ! + + 100 ! ) 9 m o d 24 (1!+2!+3!+……+100!)\equiv 9\mod24

\therefore remainder is 9 \boxed{9}

William Isoroku
Jan 14, 2015

By careful observation, we get that 1 ! + 2 ! + 3 ! + . . . . 100 ! 24 = 1 ! + 2 ! + 3 ! 24 + 4 ! + 5 ! + . . . . + 100 ! 24 \frac { 1!+2!+3!+....100! }{ 24 } =\frac { 1!+2!+3! }{ 24 } +\frac { 4!+5!+....+100! }{ 24 }

We know that any number above 4 ! 4! is divisible by 24 24 so 4 ! + 5 ! + . . . . + 100 ! 24 \frac { 4!+5!+....+100! }{ 24 } has a remainder of 0 0

We need to take care of the first fraction: f r a c 1 ! + 2 ! + 3 ! 24 frac { 1!+2!+3! }{ 24 } , which leaves a remainder of 9 \boxed{9}

Noel Lo
Mar 25, 2015

LOL the title of the question is a clue HAHAAH

Jaikirat Sandhu
Jan 14, 2015

24 is a factor of 4!, 5!, 6!.......100!. Therefore remainder is 1! +2! +3! = 1+2+6 = 9 Even if the series continued to infinity answer would have been the same.

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