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The following question was asked in a mock test for which I am preparing and I was unable to solve it.

enter image description here

What is the missing number? It must be one of these answers:

A -19
B -5
C 9
D -9

The pattern I saw was that each block is the sum of the two directly below it, divided by two. For example, 5 (the top number) = (8 + 2) / 2. Using this pattern I got -1 as the answer but it's not an option and I am unable to see any other pattern.

So, can someone please tell me how should I attempt the question?

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  • $\begingroup$ Perhaps you mistakenly used +5 instead of -5 in the last block of the third row, when calculating your provisional answer of -1? $\endgroup$ – Greg Martin Nov 16 '20 at 8:52
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The answer should be

A -19

Because

If you add two adjacent blocks and divide by 2 you get the number of the block on top of/above them

F ex. 8+2=10 ----> 10/2=5

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  • 1
    $\begingroup$ Looks like we both spotted this within moments of each other! $\endgroup$ – Stiv Nov 15 '20 at 13:08
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    $\begingroup$ we did, but I think you're right with the "trick" $\endgroup$ – Prim3numbah Nov 15 '20 at 13:18
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If the dividing line between the '9' and '?' cells is actually supposed to be positioned more to the right, then the answer is:

A) -19

Because:

The number above each pair of two adjacent cells is their arithmetical average (i.e. the mean) - e.g. for the top two rows, 5 is the average of 8 and 2.

In this case, the average of 9 and -19 yields -5 (the cell above them), as required.

However, if this is intended as a trick question and the solver is supposed to note that the -5 cell is only above a single box, then:

that box should contain the answer B) -5, as the average of one value is that value itself...!

It really depends on whether that line is positioned correctly or if it is intended as a trick question.

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  • $\begingroup$ nice, deja vu :) $\endgroup$ – Prim3numbah Nov 15 '20 at 13:07
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I think the answer is...

A(-19)

Because...

I made a drawing so that it is clearer: enter image description here The three blocks(like in the drawing) that contain the block with the question mark is outlined in red: enter image description here In this case, $a=-5$, $b=9$, and $c$ is the one represented with the question mark. Therefore, $c=2a-b=-10-9=-19$. Hence, the answer is -19(option A)

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