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Author Topic: Digital Theory Exam Question  (Read 1031 times)
N2VQ
Member

Posts: 5




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« on: October 09, 2011, 02:20:13 PM »

Choose the correct solution to the following: 0101 + 0010 =
A. 10111.
B. 1000.
C. 0111.
D. 01000.

My answer was C:
Aside from what I thought was easy binary addition
0101 in dec is 5
0010 in dec is 2
7 in bin is 0111

Why is the correct answer A Huh
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M0HCN
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Posts: 473




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« Reply #1 on: October 09, 2011, 03:23:17 PM »

Correct answer is indeed C, assuming base 2.
I cannot see a sane way to make it A.

Methinks a practice question is incorrect (It would not be the first time).

Regards, Dan.
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N2VQ
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Posts: 5




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« Reply #2 on: October 09, 2011, 03:35:12 PM »

Thanks for the reply.  That's what I was hoping.
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G3RZP
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Posts: 4842




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« Reply #3 on: October 10, 2011, 04:31:54 AM »

I make it C.
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STAYVERTICAL
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Posts: 875




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« Reply #4 on: October 14, 2011, 01:57:51 AM »

The correct answer is C.
The only way they could possibly get to A is if they made an error
in aligning the bits and assumed two 1's coincided in which case
a 1 would be carried left.
But this is certainly not the case here so it does not apply.
If they were assuming the addition was carried out by "two's complement"
addition method, they would get this answer since 1010+1101=10111
using this system.
But, this is a method which although useful for arithmetic operations inside
processors is not generally used in straightforward binary addition.

The only other thing I could possibly conjecture for why they chose A
is that they were assuming the left most bit was a sign bit, but even
in that case it would be incorrect.
Even assuming it was BCD it would not make sense to have C as the answer.
There is also the issue of big endian notation, but I am really scraping the bottom
of the barrel there!
Binary addition is simply the exclusive-or function with a carry bit to the left when two bits
co-incide and normally the right most bit is the least significant bit.

Anyway, you are right, they are wrong - at least in this universe.

73s
« Last Edit: October 14, 2011, 02:06:53 AM by STAYVERTICAL » Logged
AE4RV
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Posts: 963


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« Reply #5 on: October 14, 2011, 08:17:47 AM »

It's funny how C is still the correct answer if you assume that it is base 10 and not binary.
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