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Question of the Day - 12th August, 2009

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by quant-master » Wed Aug 12, 2009 10:36 am
Question:

What is the unit digit of 1007*1034*1221*15343*100009*10034*1091*1191*1233?

a. 6
b. 0
c. 4
d. 2
e. 8

Finding answer to this question should not take more than 15-20 secs. If you spend more than half a min than you need to check out the idea behind this question. Solution with the shortcut will be posted after 24 hrs time.

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Quant-Master
https://gmat-quants.blocked - My Blog Updated almost daily with new quant fundas. Find collection of quants question in my blog
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by prindaroy » Wed Aug 12, 2009 10:52 am
Answer is D.
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by adamsmith2009 » Wed Aug 12, 2009 10:58 am
Good question. But can we stop doing the 'solution will be posted after discussion' or 'after 24 hours'. It takes away from the what people are looking for on this site which is to solve the question, confirm the answer or learn from others the best approach to solve the question.

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by cammijc » Wed Aug 12, 2009 12:19 pm
Answer is D - 2. You multiply only the units digits, and each time drop all other numbers except the units digit.
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by tohellandback » Wed Aug 12, 2009 7:29 pm
D-2
In general- last n digits of any multiplication depend upon the multiplication of the last n digits of all the numbers.
here n happens to be 1.
The powers of two are bloody impolite!!
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Re: Question of the Day - 12th August, 2009

by quant-master » Thu Aug 13, 2009 9:06 am
quant-master wrote:Question:

What is the unit digit of 1007*1034*1221*15343*100009*10034*1091*1191*1233?

a. 6
b. 0
c. 4
d. 2
e. 8

Finding answer to this question should not take more than 15-20 secs. If you spend more than half a min than you need to check out the idea behind this question. Solution with the shortcut will be posted after 24 hrs time.

Thanks,
Quant-Master
OA is D

Like mentioned by other users above, the idea is to multiply only the unit digits of each number.

1007*1034*1221*15343*100009*10034*1091*1191*1233

multiply 7*4 =28 (forget 2, take only 8)
multiply 8*3= 24 (I ignored the 3rd number as unit digit of that number is 1 and 1 multiplied by any number is that number)
multiply 4*9 = 36
multiply 6*4 = 24
multiply 4*3 =12 (I ignored 1 again). Hence unit digit is 2

Let me know if you have any queries.

Thanks,
Quant-Master
https://gmat-quants.blocked - My Blog Updated almost daily with new quant fundas. Find collection of quants question in my blog
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