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Find the sum of all positive integers x smaller than 100 such that x^2 ? 81 is a multiple of 100

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Find the sum of all positive integers x smaller than 100 such that x^2 ? 81 is a multiple of 100. A) 200 B) 100 C) 90 D) 81 E) 50 I solved it as The possible values of x are x= 91 x = 59 x= 41 the sum of values = 191 but they said that answer is 200 !!! how ? it seems that another value x= 9 ,what did i miss here ?