Maths Olympiad Prep

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Number theory Difficulty 5.0 AIME Find the answer

How many times does 24 divide into 100! (factorial)?

A number or a short expression. Spacing and $ signs are ignored.

Solution

We first determine the number of times 2 and 3 divide into 100!=123100100!=1 \cdot 2 \cdot 3 \cdots 100. Let \langle N\rangle_{n}bethenumberoftimes be the number of times ndividesinto divides into N(i.e.wewanttofind100!24 (i.e. we want to find \langle 100!\rangle_{24}). Since 2 only divides into even integers, \langle 100!\rangle_{2}=\langle 2 \cdot 4 \cdot 6 \cdots 100\rangle.Factoringout2oncefromeachofthesemultiples,wegetthat100!2=250123502. Factoring out 2 once from each of these multiples, we get that \langle 100!\rangle_{2}=\left\langle 2^{50} \cdot 1 \cdot 2 \cdot 3 \cdots 50\right\rangle_{2}. Repeating this process, we find that \langle 100!\rangle_{2}=\left\langle 20^{50+25+12+6+3+1} \cdot 1\right\rangle_{2}=97.Similarly,100!3=333+11+3+13=48. Similarly, \langle 100!\rangle_{3}=\left\langle 3^{33+11+3+1}\right\rangle_{3}=48. Now 24=23324=2^{3} \cdot 3, so for each factor of 24 in 100! there needs to be three multiples of 2 and one multiple of 3 in 100!. Thus \langle 100!\rangle_{24}=\left(\left[\langle 100!\rangle_{2} / 3\right]+\langle 100!\rangle_{3}\right)=32,where, where [N]isthegreatestintegerlessthanorequalto is the greatest integer less than or equal to N$.

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