Question 1210371
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Candidates Q, R, and T are allowed to solve an advanced level Physics question independently. 
The probability that Q solves the question is 9/10, R solves the question is 3/7, and T solves the question is x.
(a) If the probability that at least one candidate solves the question is 19/70, find the value of x.
(b) Find the probability that:
(i) At least one candidate solves the question.
(ii) At most one candidate solves the question.
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My dear visitor, 


the problem in your post is posed INCORRECTLY.


Indeed, if the probability for Q to solve the problem is 9/10,

then the probability that "at least one candidate" will solve the problem 


            MUST  BE  AT  LEAST  9/10.



But in your post the probability that at least one candidate will solve the problem is given as 19/70,
which makes no sense, contradicts to common sense  and kills the problem to the death, instantly.



                    My condolences.