Question 1010286
Checking the first
z=(phat-p)/sqrt {(0.21)/n}
z^2*(0.21)=.0001n, squaring everything.
I agree with a.
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You can change either the error or the confidence.
Change the error as suggested to 5%
z^2*(0.21)=.0025n
n=228
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One may use any error in between.  The above is the minimum sample size given the constraints.