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I think the answer is wrong online so I would like to know what the correct answer is before I teach a student!

If it takes 6 cubes to build 3 steps, how many cubes will be needed for 11 steps?

It sounds so easy but my answer doesn't match the answer sheet. Thanks!

  • Math -

    For N steps, the number needed is
    1 + 2 + ... + N
    This sum can always be written
    N (N+1)/2

    N = 11, the number of blocks needed is
    11x12/2 = 66

  • Math -

    Thank you so much!

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