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Register now and save up to $200 Available with Beat the GMAT members only code • 5 Day FREE Trial Study Smarter, Not Harder Available with Beat the GMAT members only code • Get 300+ Practice Questions 25 Video lessons and 6 Webinars for FREE Available with Beat the GMAT members only code • 5-Day Free Trial 5-day free, full-access trial TTP Quant Available with Beat the GMAT members only code • Free Trial & Practice Exam BEAT THE GMAT EXCLUSIVE Available with Beat the GMAT members only code • Magoosh Study with Magoosh GMAT prep Available with Beat the GMAT members only code • Free Veritas GMAT Class Experience Lesson 1 Live Free Available with Beat the GMAT members only code ## Last two non zero digits of a factorial. tagged by: nisagl750 This topic has 2 expert replies and 5 member replies nisagl750 Master | Next Rank: 500 Posts Joined 31 Jan 2012 Posted: 145 messages Followed by: 2 members Upvotes: 16 #### Last two non zero digits of a factorial. Fri Feb 01, 2013 9:02 am We know the power cycle works fine to find out the last digit of any factorial (It will be zero for any factorial greater than 5) and any number raised to the power of another number. For Eg: Last non zero digit of 123456^123456 = 6 (following the power cycle of 6 which always gives a 6) I also know that last two non zero digits of a multiplication can be found out by multiplying last two digits of the numbers multiplied. i.e. last two non zero units digit of 2345*162 = 45*62 = 2790, So last two non zero digits will be 79. I wanted to know, Is there any formula to find out last two or last three non zero digits of a multiplication? for eg: last two non zero digits of 237^169? Is there any general formula or method that we can use to calculate last N non zero digits of a^b or a*b (where a & b can be any positive integers? ) ### GMAT/MBA Expert Jim@StratusPrep MBA Admissions Consultant Joined 11 Nov 2011 Posted: 2278 messages Followed by: 265 members Upvotes: 660 GMAT Score: 770 Thu Feb 07, 2013 4:27 am Point of all this --> don't waste your time with this for the GMAT. _________________ GMAT Answers provides a world class adaptive learning platform. -- Push button course navigation to simplify planning -- Daily assignments to fit your exam timeline -- Organized review that is tailored based on your abiility -- 1,000s of unique GMAT questions -- 100s of handwritten 'digital flip books' for OG questions -- 100% Free Trial and less than$20 per month after.
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nisagl750 Master | Next Rank: 500 Posts
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Thu Feb 07, 2013 10:03 am
Thanks Vineet and Jim.

I understood how to calculate. But as Jim said, I will not waste more of my time on this topic for GMAT.

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Tue Feb 05, 2013 2:36 pm
Not that I know of, but, more importantly, this is not something you will need for the gmat

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nisagl750 Master | Next Rank: 500 Posts
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Wed Feb 06, 2013 1:54 am
Thanks Jim,

Do last non zero digit problems come in GMAT?

The Iceman Master | Next Rank: 500 Posts
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Wed Feb 06, 2013 11:21 pm
This problem type is beyond the scope of GMAT, unless the problem involves a factorial of a single digit number (basically a very easy to calculate number). So, please do not invest time on such problems.

However, solely for knowledge purposes I will give you a quick formula to calculate the last non-zero integer of a factorial.

Lets say f(x) denotes the last non zero digit of factorial, then

Case 1: If tens digit of x is odd

f(x)= Last digit of (4*f[x/5]*f(Unit digit of x)); here [x/5] is the greatest integer function

Case 2:If tens digit of x is even

f(x)= Last digit of (6*f[x/5]*f(Unit digit of x)); here [x/5] is the greatest integer function

e.g. Last non zero digit of 37! can be calculated as follows:

f(37)= Last digit of (4*f[37/5]*f(7))= Last digit of (4*f(7)*f(7))= Last digit of (4*4*4) = 4

nisagl750 Master | Next Rank: 500 Posts
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Thu Feb 07, 2013 12:16 am
The Iceman wrote:
This problem type is beyond the scope of GMAT, unless the problem involves a factorial of a single digit number (basically a very easy to calculate number). So, please do not invest time on such problems.

However, solely for knowledge purposes I will give you a quick formula to calculate the last non-zero integer of a factorial.

Lets say f(x) denotes the last non zero digit of factorial, then

Case 1: If tens digit of x is odd

f(x)= Last digit of (4*f[x/5]*f(Unit digit of x)); here [x/5] is the greatest integer function

Case 2:If tens digit of x is even

f(x)= Last digit of (6*f[x/5]*f(Unit digit of x)); here [x/5] is the greatest integer function

e.g. Last non zero digit of 37! can be calculated as follows:

f(37)= Last digit of (4*f[37/5]*f(7))= Last digit of (4*f(7)*f(7))= Last digit of (4*4*4) = 4
Thanks Vineet,

To find out f(7), we have to calculate it or we can apply the above formula again.
Can you please explain giving one 3 digit number example?

-
nisagl
-----------------------------
If I make mistakes please correct me.

The Iceman Master | Next Rank: 500 Posts
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Thu Feb 07, 2013 2:38 am
nisagl750 wrote:
Thanks Vineet,

To find out f(7), we have to calculate it or we can apply the above formula again.
Can you please explain giving one 3 digit number example?

-
nisagl
-----------------------------
If I make mistakes please correct me.
Basically f(1) to f(10) are easy to find. f(3)=6=> f(4)=4=> f(5)=2=> f(6)=2=> f(7)=4=> f(8)=2=> f(9)=8=> f(10)=8

The above formula works only for two digit numbers. Let me give you a formula that works for any integer.

Consider a recursive function f(x) = Last Digit of {(2^m).f(m).f(n)}, where f(x) denotes the last non zero digit of factorial and x=5m+n

f(37)= Last Digit of {(2^7).f(7).f(2)} = last digit of (8*4*2) = 4

f(137)= Last Digit of {(2^27).f(27).f(2)} = last digit of {2^28 * f(27)} = last digit of{2^28 * last digit of(2^5).f(5).f(2)} = last digit of{2^28 * last digit of 2^7}=last digit of{2^35} = 8

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