A pyramid of blocks has 26 blocks in the bottom row and 2 fewer blocks in each successive row thereafter. How many blocks are there in the pyramid?

Arithmetic Progression
$a_1 = 26$

$a_n = 2$

$d = -2$
 

 

$a_n = a_1 + (n - 1)d$

$2 = 26 + (n - 1)(-2)$

$n = 13 ~ \text{layers of blocks}$
 

$S = \frac{1}{2}n(a_1 + a_n)$

$S = \frac{1}{2}(13)(26 + 2)$

$S = 182$