MathRevolution wrote:

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Math Revolution GMAT math practice question]

If \(n\) is the greatest positive integer for which \(5^n\) is a factor of \(50!\), what is the value of \(n\)?

\(A. 10\)

\(B. 11\)

\(C. 12\)

\(D. 13\)

\(E. 14\)

\(n \geqslant \,\,1\,\,\,\operatorname{int}\)

\(\frac{{50!}}{{{5^n}}} = \operatorname{int}\)

\(?\,\, = \,\,\max \,\,n\)

In English: how many primes equal to 5 are we able to find in the product 50*49*48*47*...*6*5*4*3*2*1 ?

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In 5 = 5*1 we find the first

In 10 = 5*2 we find the second

In 15 = 5*3 we find the third

In 20 = 5*4 we find the four

In 25 = 5*5 we find the fifth and the sixth, but forget the sixth for a moment, please!

In 30 = 5*6 we find the "sixth" (yes, that´s a lie... wait a bit!)

In 35 = 5*7 we find the "seventh" (wait...)

--- etc ---

In 50 = 5*10 we find the "tenth" (second mistake... because 5*10 = 5*5*2 .... but wait...)

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What is going on here?

When we calculate 50/5 = 10 , we find the "first" 5´s involved... with some lies...

When we divide 50 by 5^2 , we find exactly the numbers like 25 (and 50) , in which there are (at least) two 5´s in it, and only one 5 (in each case) was counted previously (between the parallel lines) ... Now they were properly counted... no more lies!

Conclusion: 50/5 + 50/25 = 12 is the right answer!

(In this case, there are no 5^3 , 5^4 , ... in 50! . In other words, each of the integers 1, 2, 3, 4, 5, ... , 50 has at most two 5´s in its corresponding prime decomposition!)

Now the "recipe" (used correctly in previous posts):

\(? = \left\lfloor {\frac{{50}}{5}} \right\rfloor + \left\lfloor {\frac{{50}}{{{5^2}}}} \right\rfloor + \left\lfloor {\frac{{50}}{{{5^3}}}} \right\rfloor + \ldots = 10 + 2 + 0 + 0 + 0 + \ldots = \boxed{12}\)

where \(\left\lfloor N \right\rfloor\) denotes the "floor" of N, that is, the greatest integer that is less than, or equal to, N.

If you prefer: when we divide 50 by 5, we have quotient 10 (the floor) and the remainder 0 (irrelevant when looking for the floor)!

Another example:

GMATH wrote:

If n is the greatest positive integer for which 3^n is a factor of 50! , what is the value of n?

A. 19

B. 20

C. 21

D. 22

E. 23

Answer:

\(? = \left\lfloor {\frac{{50}}{3}} \right\rfloor + \left\lfloor {\frac{{50}}{{{3^2}}}} \right\rfloor + \left\lfloor {\frac{{50}}{{{3^3}}}} \right\rfloor + \left\lfloor {\frac{{50}}{{{3^4}}}} \right\rfloor + \ldots = 16 + 5 + 1 + 0 + \ldots = \boxed{22}\)

Fabio, do you think it is useful to remember this "recipe" for GMAT purposes?

YES, although it´s obviously unprobable you will need it.

(This is MUCH less important than to know that the [length of the] height of the equilateral triangle is the [length of the] side times half the square root of 3, for instance.)

But... If you understood the recipe, it will be much easier to remember it (and to apply it) correctly...

That´s the GMATH´s method "essence": REAL AND DEEP UNDERSTANDING!

Regards,

fskilnik.

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Fabio Skilnik :: https://GMATH.net (Math for the GMAT) or GMATH.com.br (Portuguese version)

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