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How many ways are there to select 2 unit squares, such that one square is exactly one row above the other?

66 32 24 50

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3 solutions

We observe that for every box in the lowermost row, each of it can be paired with every other box in the second row. Hence, we get number of pairings for every box in lowest row = 6 × 4 = 24 6 \times 4 = 24 . Similarly, for every box in the middle row, each of it can be paired with any box from the topmost row, and we get the number of pairings for every box in the middle row = 4 × 2 = 8 4 \times 2 = 8 . As there is no row above topmost one, there are 0 0 possible pairings. Hence, the total number of pairings = 24 + 8 = 32 24 + 8 = 32 .

How? There is no specification that the squares, to be chosen, are to be of same size. There are 3 big squares within middle and lowest row that can be chosen for each of the 2 small squares of the topmost row.. => A total of 6 pairs. There is one one big square within the middle and top row, for each of the 6 small squares of the lowest row. => A total of 6 pairs.

and 32 pairs of squares of same size.

=> a total of 32+6+6 = 44

Ananya Aaniya - 6 years, 2 months ago

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Ma'm, there is no such option as of 44. I agree with your reasoning, but by "squares" the question meant "Individual Boxes". The question should be edited for removing ambiguity.

Venkata Karthik Bandaru - 6 years, 2 months ago

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Ya.. a few days ago i could report a problem.. these days i m not being able to see that option. though i double counted.. and gave a wrong answer of 50.. :P

Ananya Aaniya - 6 years, 2 months ago

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@Ananya Aaniya Go to the dot dot dot icon lower left corner of the problem box. There is an option to report problems.

Venkata Karthik Bandaru - 6 years, 2 months ago

Ah, I edited the question, so that it's just unit squares. Sorry!

Chung Kevin - 6 years, 2 months ago

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The question is not very cler please rectify. (I could only figure partially what the question meant from reading the comment chain)

nick baker - 6 years, 2 months ago

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@Nick Baker What do you feel is not clear about the question? Which phrase do you not understand?

Chung Kevin - 6 years, 2 months ago
David Ferreira
Mar 22, 2015

That's only ( 6 × 4 ) + ( 4 × 2 ) = 32 (6 \times 4) + (4 \times 2) = \boxed {32}

this question is not clearly stated

abhishek anand - 6 years, 2 months ago

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Can you help me with improving the problem? What do you find is confusing?

Chung Kevin - 6 years, 2 months ago

I think it is clear if you consider the word 'row'. It seems straight forward that for a square to be exactly one row above another it would have to be a unit square. Although what got me was I had it in my head that it meant the squares also had to be in line vertically!

Jesse Boldt - 6 years, 2 months ago

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I think this problem too clear, we can understand it without any difficulty...

David Ferreira - 6 years, 2 months ago
Brock Brown
Mar 27, 2015

Python: 2.7:

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rows = 3
count = 0
for row in xrange(1,rows):
    for column in xrange(row*2):
        count += (row + 1) * 2
print "Answer:", count

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