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# Quant - Number of digits in a number

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Intern
Joined: 28 Aug 2012
Posts: 7

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Quant - Number of digits in a number [#permalink]

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30 Sep 2012, 08:12
Hi,

Is there any shortcut to find out number of digits in a number say (4^34)(5^38) ?
Help much appreciated.

Thanks,
Weirdo

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Director
Joined: 22 Mar 2011
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Re: Quant - Number of digits in a number [#permalink]

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30 Sep 2012, 14:01
Weirdo2989 wrote:
Hi,

Is there any shortcut to find out number of digits in a number say (4^34)(5^38) ?
Help much appreciated.

Thanks,
Weirdo

Not a shortcut, but this is how I would do it: I would find the positive integer $$n$$ for which $$10^n<(4^{34})(5^{38})<10^{n+1}$$.

$$4^{34}\cdot{5^{38}}=2^{68}\cdot{5^{38}}=10^{38}\cdot{2^{30}}$$.
$$2^{30}=(2^{10})^3=1024^3>1000^3=10^9.$$
Also, $$2^{30}=2^{10}\cdot{2^{20}}=2^{10}\cdot4^{10}<2^{10}\cdot5^{10}=10^{10}$$.

Our number is between $$10^{38+9}$$ and $$10^{38+10}$$, therefore we can conclude that it has $$48$$ digits ($$10^{47}$$ has $$48$$ digits and $$10^{48}$$ has $$49$$ digits).
_________________

PhD in Applied Mathematics
Love GMAT Quant questions and running.

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Intern
Joined: 28 Aug 2012
Posts: 7

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Re: Quant - Number of digits in a number [#permalink]

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01 Oct 2012, 00:16
Simply amazing.. Thanks a ton

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Re: Quant - Number of digits in a number   [#permalink] 01 Oct 2012, 00:16
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