Question 465515
{{{p=mu=80/400=0.2}}}, q=0.8, n=400
{{{sigma=sqrt(pq/n)=sqrt((.2*.8)/400)=.02}}}
(a,b)= {{{p+-z[alpha 2]}}}*{{{sigma}}}=.2+-(2.576*.02)=(.1485, .2515)
We can say with 99% confidence that the proportion of cars costing over $700,000 is in the interval  (.1485, .2515).
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Ed