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math

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a man saved a sum of 200 dinars in the first month, 250 in the second month, 300 in the third month and he continue to save at the streght. after how many month can he buy a radio instronment coastin 13500 dinars

  • math -

    arithmetic series
    a = 200
    d = 50

    Sum(n) = (n/2)[2a + (n-1)d]
    13500 = n/2 [400 + 50(n-1)]
    27000 = n(50n + 350)
    50n^2 + 350n - 27000 = 0
    10n^2 + 7n - 5400 = 0
    n = ..... One answer will be + , the other -, reject the -

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