Triangle sums


Each cell in this triangle is the sum of the two cells below it. Can you complete them?
1) triangle 1*

75
43 .
28 . .
18 . . .
. . . . 8

I first encountered this type of puzzle in the Dutch translation of “One minute puzzles”, published by Arcturus Publishing Limited, London. This book published the numbers in circles, and all puzzles had the difficulty level of the one above, where there is always at least one cell which can be calculated with a simple addition or subtraction.
I replaced the circles with the pyramid pictured above, and in the following puzzles you will find an extra difficulty level introduced.

2) triangle 2*

.
. .
18 . 15
. . . 6
7 . . . 2

3) triangle 3*

115
. .
19 . 36
. . . .
7 . . . 8

4) triangle 4*

80
. .
18 . 24
. . . .
. 1 . 4 .

This type of puzzles exercises the parts of your brain which performs the arithmetic. If I interpret this article correctly, that is the horizontal segment of the bilateral intraparietal sulcus (HIPS), together with the precentral sulcus and inferior frontal gyrus.

You can check your solutions at solution 221, solution 231, solution 241, and solution 212.

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