A pile of blocks has 40 blocks in the bottom row 36 blocks in the second row 32 blocks in the third row and so on until there are only 4 blocks on the top row how many blocks? Are there in the 7th row? How many blocks are there in all

Answer 1

(40-4)/4 = 9
so, there are 9 rows after the 1st, making 10 in all.

On the nth row, there are
40-(n-1)*4 = 44-4n bricks

The sum of n rows, Sn, with
a = 40
d = -4

is

Sn = n/2 (2*40 - (n-1)*4) = 2n(21-n)

Now you can answer the questions.

Answer 2

135