# What is the missing value in this number pyramid?

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

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

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

• Looks like we both spotted this within moments of each other! – Stiv Nov 15 '20 at 13:08
• we did, but I think you're right with the "trick" – Prim3numbah Nov 15 '20 at 13:18

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.

• nice, deja vu :) – Prim3numbah Nov 15 '20 at 13:07

I made a drawing so that it is clearer: The three blocks(like in the drawing) that contain the block with the question mark is outlined in red: 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)