The probability that a student passes at least in one of the three examinations A,B,C is 0.75.

The probability that he passes in at least two of the exams is 0.5.

And the prabability he passes in exactly two of the exams is 0.4.

Denote \( p,q,r\) as the probabilities of the student passing in A,B,C respectively.

Let \(p+q+r=\frac xy \) for coprime positive integers \(x,y\), find the value of \(x-y\).

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