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M32-01

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M32-01 [#permalink]

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Re M32-01 [#permalink]

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New post 21 May 2016, 04:21
Official Solution:

How many trailing zeroes does \(49! + 50!\) have?

A. 9
B. 10
C. 11
D. 12
E. 22


Factor out 49! from the expression: \(49! + 50!=49!(1+50)=49!*51\).

51 won't contribute to the number of zeros at the end of the number, therefore all zeros will come from 49!.

Trailing zeros in 49!: \(\frac{49}{5}+\frac{49}{5^2}=9+1=10\)

THEORY:

Trailing zeros:

Trailing zeros are a sequence of 0's in the decimal representation of a number, after which no other digits follow.

For example 125,000 has 3 trailing zeros;

The number of trailing zeros n!, the factorial of a non-negative integer \(n\), can be determined with this formula:

\(\frac{n}{5}+\frac{n}{5^2}+\frac{n}{5^3}+...+\frac{n}{5^k}\), where \(k\) must be chosen such that \(5^{(k+1)} \gt n\)

It's easier if we consider an example:

How many zeros are in the end (after which no other digits follow) of 32!?

\(\frac{32}{5}+\frac{32}{5^2}=6+1=7\). Notice that the last denominator (\(5^2\)) must be less than 32. Also notice that we take into account only the quotient of the division, that is \(\frac{32}{5}=6\).

So there are 7 zeros in the end of 32!.

Another example, how many trailing zeros does 125! have?

\(\frac{125}{5}+\frac{125}{5^2}+\frac{125}{5^3}=25+5+1=31\),

The formula actually counts the number of factors 5 in n!, but since there are at least as many factors 2, this is equivalent to the number of factors 10, each of which gives one more trailing zero.


Answer: B
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Re: M32-01 [#permalink]

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New post 28 May 2016, 10:10
Hello,
Please help me out in understanding why the answer is 10 and not 11.
49! has 10 trailing zeros OK.
But 49!(10*5+1)? doesn't the 10 in the expression add an extra zero to the trailing zeros of 49! ?

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Re: M32-01 [#permalink]

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New post 28 May 2016, 10:19
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Avigano wrote:
Hello,
Please help me out in understanding why the answer is 10 and not 11.
49! has 10 trailing zeros OK.
But 49!(10*5+1)? doesn't the 10 in the expression add an extra zero to the trailing zeros of 49! ?


Hi,
no, 49!(10*5+1) = 49!(50+1) = 49!*51...
Now 51 does not contain any 5 in it, so the term depends on ONLY 49! and as you have said 49! has 10 5s
so ans is 10
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Re: M32-01 [#permalink]

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New post 28 May 2016, 10:38
chetan2u wrote:
Avigano wrote:
Hello,
Please help me out in understanding why the answer is 10 and not 11.
49! has 10 trailing zeros OK.
But 49!(10*5+1)? doesn't the 10 in the expression add an extra zero to the trailing zeros of 49! ?


Hi,
no, 49!(10*5+1) = 49!(50+1) = 49!*51...
Now 51 does not contain any 5 in it, so the term depends on ONLY 49! and as you have said 49! has 10 5s
so ans is 10


Ok! got it! thanks

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Re: M32-01 [#permalink]

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New post 29 Dec 2016, 14:13
chetan2u wrote:
Avigano wrote:
Hello,
Please help me out in understanding why the answer is 10 and not 11.
49! has 10 trailing zeros OK.
But 49!(10*5+1)? doesn't the 10 in the expression add an extra zero to the trailing zeros of 49! ?


Hi,
no, 49!(10*5+1) = 49!(50+1) = 49!*51...
Now 51 does not contain any 5 in it, so the term depends on ONLY 49! and as you have said 49! has 10 5s
so ans is 10



Hi All - I don't 100% understand the rationale behind 51 not contributing to the number of trailing zeros. Does 51 not contain 10^1? Thanks.

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Re: M32-01 [#permalink]

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New post 29 Dec 2016, 22:28
Dear Bunuel,

I know the concept and deduced the final step 49! * 51. But 49/5 = 9.8 ~ 10 and 49/25 =1.96 ~ 2 .So I marked the answer as 12 instead of 10.Please clarify,if we need to select the lowest rounded integer or do we need to consider the nearest rounded integer to answer these kind of questions.

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Re: M32-01 [#permalink]

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New post 30 Dec 2016, 00:37
GouthamNandu wrote:
Dear Bunuel,

I know the concept and deduced the final step 49! * 51. But 49/5 = 9.8 ~ 10 and 49/25 =1.96 ~ 2 .So I marked the answer as 12 instead of 10.Please clarify,if we need to select the lowest rounded integer or do we need to consider the nearest rounded integer to answer these kind of questions.


Yes, you should take only the quotient into the account, that is 49/5 = 9 and 49/25 = 1.



Check Trailing Zeros Questions and Power of a number in a factorial questions in our Special Questions Directory.

Hope it helps.
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Re: M32-01 [#permalink]

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New post 04 Jan 2017, 14:55
chetan2u wrote:
Avigano wrote:
Hello,
Please help me out in understanding why the answer is 10 and not 11.
49! has 10 trailing zeros OK.
But 49!(10*5+1)? doesn't the 10 in the expression add an extra zero to the trailing zeros of 49! ?


Hi,
no, 49!(10*5+1) = 49!(50+1) = 49!*51...
Now 51 does not contain any 5 in it, so the term depends on ONLY 49! and as you have said 49! has 10 5s
so ans is 10


Hi. How does 51 not contain any 5s?

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Re: M32-01 [#permalink]

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New post 05 Jan 2017, 03:23
Cez005 wrote:
chetan2u wrote:
Avigano wrote:
Hello,
Please help me out in understanding why the answer is 10 and not 11.
49! has 10 trailing zeros OK.
But 49!(10*5+1)? doesn't the 10 in the expression add an extra zero to the trailing zeros of 49! ?


Hi,
no, 49!(10*5+1) = 49!(50+1) = 49!*51...
Now 51 does not contain any 5 in it, so the term depends on ONLY 49! and as you have said 49! has 10 5s
so ans is 10


Hi. How does 51 not contain any 5s?


51 = 3*17, so he means that 51 does not have 5 as a factor.
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New post 20 Jan 2017, 10:16
I think this is a high-quality question.

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Re: M32-01 [#permalink]

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New post 17 Mar 2017, 22:05
Sorry i have a question. what is wrong if you factor 49 and 50 factorial and add??
So you will get 22

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Re: M32-01 [#permalink]

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New post 15 Jul 2017, 13:08
Hi,

I solved with the trailing zero method only but I calculated for 49! and 50! separately and I got 22 as answer. I want to know how that is a wrong method of solving. Thanks!
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New post 16 Jul 2017, 04:14
ashikaverma13 wrote:
Hi,

I solved with the trailing zero method only but I calculated for 49! and 50! separately and I got 22 as answer. I want to know how that is a wrong method of solving. Thanks!


With that logic the number of trailing zeros of 10 + 100 would be 1 + 2 = 3 but in fact 10 + 100 = 110 and 110 has 1 trailing zero.

Hope that the above example answers your question.
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Re: M32-01 [#permalink]

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New post 30 Sep 2017, 10:04
This was confusing for me too, but the logic lies in the fact that 49! is a factorial. 1x2x3x4....40x41x...49. vs. 51 which is just 17x3. There are no 5s in 51. The formula allows you to count how many 5s are in factorials. If it was 51! (factorial) then yes it would have 12 fives, but the answer is 10 because 51 is NOT a factorial in this problem (after you simplify it.)

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Re: M32-01   [#permalink] 30 Sep 2017, 10:04
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