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Math

posted by on .

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?

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

  • Math - ,

    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

  • Math - ,

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

    Thanks a lot

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