Stack the Stones
Read the puzzle
A brick pyramid where each brick equals the sum of the two bricks beneath it (example: 13 sits on 5 and 8). Bottom row, left to right: 5, 8, ?, ?. Second row: 13, ?, 9. Third row: ?, 20. Top: ?. Which number belongs on the top brick?
Which number belongs on the top brick?
44
20 - 9 = 11, 13 + 11 = 24, then 24 + 20 = 44.
How to solve it
Work from the bricks you know, using the rule that each brick is the sum of the two below it.
The 20 in the third row sits on the middle brick of the second row and the 9. So the middle brick is 20 - 9 = 11.
The empty brick in the third row sits on 13 and that 11, so it is 13 + 11 = 24.
The top brick sits on 24 and 20, so it is 24 + 20 = 44.
The two missing bottom bricks are not needed, but they check out: 8 + 3 = 11 and 3 + 6 = 9, so the bottom row is 5, 8, 3, 6, and 5 + 8 = 13 matches too.
The usual slip is starting from the bottom row, where two bricks are missing, and getting stuck, or adding 20 and 9 to get 29 for the middle brick instead of subtracting. Quick check: the top also equals the second row's outer bricks plus twice its middle brick, 13 + 11 + 11 + 9 = 44.
