in Quantitative Aptitude edited by
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5 votes
5 votes

How many proper divisors (that is, divisors other than $1$ or $7200$)  does $7200$ have ?

  1. $18$
  2. $20$
  3. $52$
  4. $54$
  5. $60$
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2 Answers

9 votes
9 votes
Best answer

$7200$ can be written as $2^{5} * 3^{2} * 5^{2}$

Therefore, there are $(5+1) * ( 2+1) * (2+1) = 54$ divisors including $1$, and $7200$.

Hence, the total divisors (excluding $1$ and $7200$) = $54-2 = 52$

Here's some justification to support the above method : 

Let $n\in\mathbb{N}$. Then by fundamental theorem of arithmetic we can write $n\in \mathbb{N}, n\neq 1$ by $n=p_1^{a_1}p_2^{a_2}\dots p_k^{a_k}$ where $p_1,p_2,\dots p_k$ are prime and $a_k\in\mathbb{N}$, $k=1,2,...,k$. Hence, number of divisors of $n$ =  $(a_1+1)(a_2+1)\cdots (a_k+1)$.

More on this here.

Answer ( C )

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3 Comments

provide the justification for the formulas, or atleast specify resources...

otherwise we have memorize it without learning the concept
3
3

@Madhu Bhargava

thank you for providing justification, but one more small doubt that, how can you quickly identified 7200 = 32*9*25 ?

otherwise just followed LCM method ?

0
0
we can decompose as follows

$7200 = 72 * 100 = 8*9*10*10 = 2^3*3^2*5*2*5*2 = 2^5 * 3^2 * 5^2$
2
2
0 votes
0 votes
7200=2^5 *3^2 * 5^2

total divisors=(5+1)*(2+1)*(2+1)=54

but 1 and 7200 excluding

so remaining divisors are 54-2=52

so ans is (C).
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