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- Iggy RockoLv 71 month ago
50 + 60 + 70 + ... = 950

Let n be the number of floors.

40 + 10(1) + 40 + 10(2) + 40 + 10(3) + ... + 40 + 10(n) = 950

40n + 10(1 + 2 + ... + n) = 950

...............n

40n + 10∑ i = 950

..............i=1

40n + 10(n(n + 1))/2 = 950

40n + 5n^2 + 5n = 950

5n^2 + 45n = 950

n^2 + 9n = 190

n^2 + 9n - 190 = 0

(n + 19)(n - 10) = 0

n = 10 or n = -19

A negative number of floors makes no sense so discard n = -19, leaving just

n = 10

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