Question 1206319
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As with other today's posts (and, generally, with many other posts at this forum),
the problem formulation is INCORRECT.


It is incorrect, because it is incomplete.


To be complete, it must say in the condition, that populations S1 and S2 are disjoint (= mutually exclusive).


The fact that  P(S1) + P(S2) = 0.7 + 0.3 = 1  does not imply automatically that  S1  and  S2  are disjoint, 
since they still may have intersection  (for example,  S2  can be a part of  S1).



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Edwin, I do not read in the head of the instructor.

I even do not try to do it . . . 

I read what I see in the post.