Question 1178386
Std error is sqrt(0.5*0.5/2295), with no other data being given, this is the most conservative result.
it is 0.0104
the margin of error for a 95%CI is z(0.95)*SE=0.020; the 0.0172 would work for using 50% probability and a 90% CI
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Now given that it was 46%, one can recompute the SE as 0.01780 for a 90% Ci.
90%CI is (0.4422, 0.4778)
I would agree that the percentage of voters opposed is likely to be 50%, because the 90% CI for approval does not contain 50%, so therefore the 90% CI for opposition WILL contain 50%.

Two comments: the first part does not give a point estimate, so one has to use 50%.  If it did give a point estimate, then the value would change for the CI.
Second, the value algebraically and on the calculator will be different, because the algebraic proportion can have a fraction in the numerator (half of 2295 is 1147.5.)  But the calculator will require an integer, and to four decimal places, the result will be different.