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Re: If r, s, and t are all positive integers, what is the [#permalink]
02 Aug 2012, 15:19

2

This post received KUDOS

Expert's post

1

This post was BOOKMARKED

If r, s, and t are all positive integers, what is the remainder when 2^(rst) is divided by 10?

First of all, when a positive integer is divided by 10, the remainder is the units digit of that integer. For example, 30 divided by 10 yields the remainder of 0, 31 divided by 10 yields the remainder of 1, 32 divided by 10 yields the remainder of 2, ...

Next, the units digit of 2 in positive integer power repeats in blocks of 4: {2, 4, 8, 6}

The units digit of 2^1 is 2; The units digit of 2^2 is 4; The units digit of 2^3 is 8; The units digit of 2^4 is 6; The units digit of 2^5 is 2, AGAIN; ...

(1) s is even --> rst is even, hence the units digit of 2^(rst) is either 4 or 6. Not sufficient.

(2) rs = 4 --> rst is a multiple of 4, hence the units digit of 2^(rst) is the same as the units digit of 2^4 so 6, which means that the remainder upon division of 2^(rst) by 10 is 6. Sufficient.

Re: If r, s, and t are all positive integers, what is the [#permalink]
11 Aug 2013, 04:39

r,s,t are +ve

REM(2^rst/10) ?

(1).

s is even also even * even = even and even*odd=even

But REM(2^2/10) and REM(2^4/10) are different hence insufficient .

(2).

rs=4

REM(2^4t/10)

REM(2^4/10) ....REM(2^8/10).......REM(2^12/10) .... All are same

Hence sufficient

(B). it is ! _________________

Rgds, TGC! _____________________________________________________________________ I Assisted You => KUDOS Please _____________________________________________________________________________

Re: If r, s, and t are all positive integers, what is the remain [#permalink]
19 Aug 2013, 05:42

a) if s is even, i.e. rst = even -> 2^even/10 -> can't determine b) rs = 4, i.e. rst = 4t -> 2^4t/10 -> 2^4t will always have 6 in unit's place(always the multiplication for unit place will be 6*6), so remainder will be 6 -> determined.

Re: If r, s, and t are all positive integers, what is the [#permalink]
09 Mar 2014, 15:40

Any integer that does not end in 0 will have a positive remainder when divided by 10. Specifically, the remainder will be equal to the ones column. No power of 2 ends in 0. We need the units digit of 2^(rst).

Re: If r, s, and t are all positive integers, what is the [#permalink]
12 Jun 2014, 19:11

If r, s, and t are all positive integers, what is the remainder of 2^p/10, if p = rst?

(1) s is even

(2) p = 4t

Hi everyone, i have a doubt with statement B. since p=4t so when divided by 10 we can cancel a 2 from both numerator and denominator so we have 2^3t/5 which is 8^t/5 so in this case we have different remainders each time.

Re: If r, s, and t are all positive integers, what is the [#permalink]
13 Jun 2014, 00:37

Expert's post

snehamd1309 wrote:

If r, s, and t are all positive integers, what is the remainder of 2^p/10, if p = rst?

(1) s is even

(2) p = 4t

Hi everyone, i have a doubt with statement B. since p=4t so when divided by 10 we can cancel a 2 from both numerator and denominator so we have 2^3t/5 which is 8^t/5 so in this case we have different remainders each time.

Please advice.

\(\frac{2^{4t}}{10}=\frac{2^{4t-1}}{5}\) not 2^3t/5. Also, when we are asked to find the remainder of a/b it's not correct to reduce the fraction and find the remainder of the resulting fraction. For example, the remainder when 15 is divided by 6 is 3, but if you reduce that by 3 and find the remainder of 5 by 2 you'd get the remainder of 1.

Re: If r, s, and t are all positive integers, what is the [#permalink]
13 Jun 2014, 02:19

Bunuel wrote:

snehamd1309 wrote:

If r, s, and t are all positive integers, what is the remainder of 2^p/10, if p = rst?

(1) s is even

(2) p = 4t

Hi everyone, i have a doubt with statement B. since p=4t so when divided by 10 we can cancel a 2 from both numerator and denominator so we have 2^3t/5 which is 8^t/5 so in this case we have different remainders each time.

Please advice.

\(\frac{2^{4t}}{10}=\frac{2^{4t-1}}{5}\) not 2^3t/5. Also, when we are asked to find the remainder of a/b it's not correct to reduce the fraction and find the remainder of the resulting fraction. For example, the remainder when 15 is divided by 6 is 3, but if you reduce that by 3 and find the remainder of 5 by 2 you'd get the remainder of 1.

Hope its clear.

Thanks Bunuel for your reply. I understood that one should not cancel out however cant understand 2^4t/10 is simplified into 2^4t-1/5 and not 2^3t/5. Don't we cancel the powers. for Example 2^3/2= 2^2. then why cant it be in the previous one.Please help.Thanks

Re: If r, s, and t are all positive integers, what is the [#permalink]
13 Jun 2014, 02:27

Expert's post

snehamd1309 wrote:

Bunuel wrote:

snehamd1309 wrote:

If r, s, and t are all positive integers, what is the remainder of 2^p/10, if p = rst?

(1) s is even

(2) p = 4t

Hi everyone, i have a doubt with statement B. since p=4t so when divided by 10 we can cancel a 2 from both numerator and denominator so we have 2^3t/5 which is 8^t/5 so in this case we have different remainders each time.

Please advice.

\(\frac{2^{4t}}{10}=\frac{2^{4t-1}}{5}\) not 2^3t/5. Also, when we are asked to find the remainder of a/b it's not correct to reduce the fraction and find the remainder of the resulting fraction. For example, the remainder when 15 is divided by 6 is 3, but if you reduce that by 3 and find the remainder of 5 by 2 you'd get the remainder of 1.

Hope its clear.

Thanks Bunuel for your reply. I understood that one should not cancel out however cant understand 2^4t/10 is simplified into 2^4t-1/5 and not 2^3t/5. Don't we cancel the powers. for Example 2^3/2= 2^2. then why cant it be in the previous one.Please help.Thanks

\(\frac{a^n}{a^m}=a^{n-m}\). Hence, \(\frac{2^3}{2^2}=2^{3-2}=2\) the same way: \(\frac{2^{4t}}{2}=2^{4t-1}\).

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