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Re: Everything about Factorials on the GMAT [#permalink]
11 Jun 2014, 12:18

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 denominators must be less than or equal to 32 also notice that we take into account only the quotient of division (that is \frac{32}{5}=6 not 6.4). Therefore, 32! has 7 trailing zeros.

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. how come we divide 32 by 5?

Re: Everything about Factorials on the GMAT [#permalink]
11 Jun 2014, 13:13

Expert's post

sagnik2422 wrote:

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 denominators must be less than or equal to 32 also notice that we take into account only the quotient of division (that is \frac{32}{5}=6 not 6.4). Therefore, 32! has 7 trailing zeros.

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. how come we divide 32 by 5?

Well, this is done according to the formula given there. _________________

Re: Everything about Factorials on the GMAT [#permalink]
22 Apr 2015, 00:07

A more generalised method for finding powers of non prime numbers in a factorial:

Assuming we have to find the power of 12 in 30!

Now, 30 = 2 * 3 * 5.

Find what the highest power of each prime is in the given factorial (by the same method as given by Bunuel)

We have 25 + 12 + 6 + 3 + 1 = 47 2s in 50!

16 + 5 + 1 = 22 3s in 50!

10 + 2 = 12 5s in 50!

We need a combination of one 2, one 3 and one 5 to get 30. In the given factorial we have only 12 5s. Hence only twelve 30s are possible. So the greatest power of 30 in 50! is 12.

Similarly we can find powers of 4 (number of pairs of 2) etc.

Re: Everything about Factorials on the GMAT [#permalink]
26 May 2015, 11:51

Bunuel wrote:

noboru wrote:

It's better to illustrate it on the example: How many powers of 900 are in 50! \(900=2^2*3^2*5^2\)

Find the power of 2: \(\frac{50}{2}+\frac{50}{4}+\frac{50}{8}+\frac{50}{16}+\frac{50}{32}=25+12+6+3+1=47\)

= \(2^{47}\)

Find the power of 3: \(\frac{50}{3}+\frac{50}{9}+\frac{50}{27}=16+5+1=22\)

=\(3^{22}\)

Find the power of 5: \(\frac{50}{5}+\frac{50}{25}=10+2=12\)

=\(5^{12}\)

We need all of them (2,3,5) to be represented twice in 900, 5 can provide us with only 6 pairs, thus there is 900 in the power of 6 in 50! 900^6

If I go to excel and calculate factorial 50! and then divide by 900^6, i still have room for 900^7 which then still yields a positive result. Could you explain that further?

Re: Everything about Factorials on the GMAT [#permalink]
27 May 2015, 02:34

1

This post received KUDOS

Expert's post

reto wrote:

Bunuel wrote:

noboru wrote:

It's better to illustrate it on the example: How many powers of 900 are in 50! \(900=2^2*3^2*5^2\)

Find the power of 2: \(\frac{50}{2}+\frac{50}{4}+\frac{50}{8}+\frac{50}{16}+\frac{50}{32}=25+12+6+3+1=47\)

= \(2^{47}\)

Find the power of 3: \(\frac{50}{3}+\frac{50}{9}+\frac{50}{27}=16+5+1=22\)

=\(3^{22}\)

Find the power of 5: \(\frac{50}{5}+\frac{50}{25}=10+2=12\)

=\(5^{12}\)

We need all of them (2,3,5) to be represented twice in 900, 5 can provide us with only 6 pairs, thus there is 900 in the power of 6 in 50! 900^6

If I go to excel and calculate factorial 50! and then divide by 900^6, i still have room for 900^7 which then still yields a positive result. Could you explain that further?

Re: Everything about Factorials on the GMAT [#permalink]
22 Sep 2015, 14:58

Bunuel wrote:

noboru wrote:

Point 1 is just point 2 for k=5, isnt it?

Yes, it is. I've separated them as for GMAT generally we need only trailing zeros and almost never other prime's power.

shalva wrote:

Can you please post this one too? It's still interesting, though may not be usable for GMAT.

It's better to illustrate it on the example: How many powers of 900 are in 50! \(900=2^2*3^2*5^2\)

Find the power of 2: \(\frac{50}{2}+\frac{50}{4}+\frac{50}{8}+\frac{50}{16}+\frac{50}{32}=25+12+6+3+1=47\)

= \(2^{47}\)

Find the power of 3: \(\frac{50}{3}+\frac{50}{9}+\frac{50}{27}=16+5+1=22\)

=\(3^{22}\)

Find the power of 5: \(\frac{50}{5}+\frac{50}{25}=10+2=12\)

=\(5^{12}\)

We need all of them (2,3,5) to be represented twice in 900, 5 can provide us with only 6 pairs, thus there is 900 in the power of 6 in 50! 900^6

Can someone please explain why 900 is there 6 times versus 12 ?

there are 12 5's in 50!

30 = 2 * 3 * 5 30 x 30 = 900 = 2^2 x 3^2 x 5^2

I understand that 900 is a pair of the primes 2 , 3 and 5 but if there are a total of 12 5's then 6 would be in the first 30 and 6 in the next 30 . 6 + 6 = 12 so 900 ^ 12 would seem more correct?

Re: Everything about Factorials on the GMAT [#permalink]
04 Nov 2015, 03:16

ayushkhatri wrote:

Bunuel wrote:

noboru wrote:

Point 1 is just point 2 for k=5, isnt it?

Yes, it is. I've separated them as for GMAT generally we need only trailing zeros and almost never other prime's power.

shalva wrote:

Can you please post this one too? It's still interesting, though may not be usable for GMAT.

It's better to illustrate it on the example: How many powers of 900 are in 50! \(900=2^2*3^2*5^2\)

Find the power of 2: \(\frac{50}{2}+\frac{50}{4}+\frac{50}{8}+\frac{50}{16}+\frac{50}{32}=25+12+6+3+1=47\)

= \(2^{47}\)

Find the power of 3: \(\frac{50}{3}+\frac{50}{9}+\frac{50}{27}=16+5+1=22\)

=\(3^{22}\)

Find the power of 5: \(\frac{50}{5}+\frac{50}{25}=10+2=12\)

=\(5^{12}\)

We need all of them (2,3,5) to be represented twice in 900, 5 can provide us with only 6 pairs, thus there is 900 in the power of 6 in 50! 900^6

Can someone please explain why 900 is there 6 times versus 12 ?

there are 12 5's in 50!

30 = 2 * 3 * 5 30 x 30 = 900 = 2^2 x 3^2 x 5^2

I understand that 900 is a pair of the primes 2 , 3 and 5 but if there are a total of 12 5's then 6 would be in the first 30 and 6 in the next 30 . 6 + 6 = 12 so 900 ^ 12 would seem more correct?

ayushkhatri In the prime factorization of 900 , the highest power of 5 is 2 . 900=\(2^{2}\)∗\(3^{2}\)∗\(5^{2}\)

as per the calculations in the previous posts , we know power of 5 in 50! is 12 . So the 12 powers of 5 can be divided among 6 pairs . We divide in pairs because 900 has 5^2 .

Just think for a moment , if it was indeed 900 ^ {12} , then we will have (5^2)^12 that is 5^24 . There will be 12 powers of 30 in 50! . _________________

When everything seems to be going against you, remember that the airplane takes off against the wind, not with it. - Henry Ford +1 Kudos if you find this post helpful

gmatclubot

Re: Everything about Factorials on the GMAT
[#permalink]
04 Nov 2015, 03:16

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