Factorial !!!!!!

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Find the remainder when 1! +2! +3! +4! +5! +.................100! is divided by 24.


The answer is 9.

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1 solution

Garvit Satija
Jun 1, 2015

Given : 1 ! + 2 ! + 3 ! + . . . . . . . . + 100 ! 24 \frac{1!+2!+3!+........+100!}{24}

Now it can also be written as 1 24 \frac{1}{24} + 1 2 24 \frac{1*2}{24} + 1 2 3 24 \frac{1*2*3}{24} + 1 2 3 4 24 \frac{1*2*3*4}{24} + ............ + 1 2 3 4 5 6 . . . . . . 100 24 \frac{1*2*3*4*5*6*......*100}{24}

But 4! =24

Therefore every no. after 4! will give a quotient and will leave a remainder 0 ( For example 5! will give quotient 5 and remainder zero ).

This means that remainder will be equal to 1! + 2! + 3! which is 1 + 2 + 6 = 9 \boxed{1 + 2+ 6 = 9}

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