A pile of bricks has 85 bricks in the bottom row, 81 bricks in the second row up, 77 in the third, and so on up to the top row that contains only 1 brick. How many bricks are in the 12th row?

Question

I don't know if I'm right but I attempted this problem below

c = a1 - d
c = 85 - 4 = 81

- 85 + 81 = 166

Sn = n / 2 (a1 + an)

Sn = 85 / 2 (85 + 166)

Sn = 10667.5

Answer 1

I can't follow you calculation.

Row 1 has

85

Row 2 has

81

row 3 has
77

The nth row will have

85-4(n-1)

so if n=12

then

85-4(12-1)=85-44=41 bricks

Answer 2

Right, instead of using the Sn formula. I could have just use an = a1 + (n - 1) d

Thanks a lot