conflicting answer

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san2009: Master | Next Rank: 500 Posts; Posts: 171; Joined: Fri Apr 16, 2010 1:02 am; Thanked: 1 times

by san2009 » Fri May 07, 2010 6:35 am

f n is a positive integer, what is the remainder when 38n+3 + 2 is divided by 5?

A. 0
B. 1
C. 2
D. 3
E. 4

I get answer E, but the online blogs have also given answer as A i.e. remainder 0.

Advise.

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Source: Beat The GMAT — Problem Solving | Fri May 07, 2010 6:35 am

analyst218: Master | Next Rank: 500 Posts; Posts: 140; Joined: Fri Feb 05, 2010 2:43 pm; Thanked: 3 times; GMAT Score:720

by analyst218 » Fri May 07, 2010 9:38 am

san2009 wrote:f n is a positive integer, what is the remainder when 38n+3 + 2 is divided by 5?

A. 0
B. 1
C. 2
D. 3
E. 4

I get answer E, but the online blogs have also given answer as A i.e. remainder 0.

Advise.

there must be something missing in the question
n=1 r=3
n=2 r=1
n=3 r=4

"if n is a positive integer" doesn't give you the answer..

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san2009: Master | Next Rank: 500 Posts; Posts: 171; Joined: Fri Apr 16, 2010 1:02 am; Thanked: 1 times

by san2009 » Fri May 07, 2010 11:31 am

apologies
the expression is
3^(8n+3) + 2

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analyst218: Master | Next Rank: 500 Posts; Posts: 140; Joined: Fri Feb 05, 2010 2:43 pm; Thanked: 3 times; GMAT Score:720

by analyst218 » Fri May 07, 2010 12:10 pm

san2009 wrote:apologies
the expression is
3^(8n+3) + 2

ok the answer has to be A. 0
since for every postive integer n
expression 3^8n+3 will yield units digit of 3.
since you add +2 for everytime the number will end in units digit of 5.
hence remainder =0 when divided by 5.

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san2009: Master | Next Rank: 500 Posts; Posts: 171; Joined: Fri Apr 16, 2010 1:02 am; Thanked: 1 times

by san2009 » Fri May 07, 2010 12:22 pm

i don't follow. For example, when you plug in n=1, n=2, n=3,
the expression 3^(8n+3) gives you a units digit of 7 (and not 3 as you pointed out)
then, when you add in the 2, your units digit becomes 9
when you divide 9 by 5, you get a remainder of 4
which is answer choice E

is their an instructor/moderator who could mediate?

Thanks

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analyst218: Master | Next Rank: 500 Posts; Posts: 140; Joined: Fri Feb 05, 2010 2:43 pm; Thanked: 3 times; GMAT Score:720

by analyst218 » Fri May 07, 2010 12:32 pm

san2009 wrote:i don't follow. For example, when you plug in n=1, n=2, n=3,
the expression 3^(8n+3) gives you a units digit of 7 (and not 3 as you pointed out)
then, when you add in the 2, your units digit becomes 9
when you divide 9 by 5, you get a remainder of 4
which is answer choice E

is their an instructor/moderator who could mediate?

Thanks

you're right.
I added 2 both in the exponent and to the expression.
E.

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