Wild-type fruit flies have red eyes, but a recessive mutation produces white-eyed individuals

Wild-type fruit flies have red eyes, but a recessive mutation produces white-eyed individuals. A researcher wants to assess the frequency of heterozygous individuals among red-eyed fruit flies. A heterozygous red-eyed fly crossed with a white-eyed mutant will have mixed progeny. Of the 100 red-eyed fruit flies crossed with white-eyed mutants, 11 produced mixed progeny.
 (a)Show that the conditions for the large-sample confidence interval are not met. Show that the conditions for the plus four interval are met.
(b) Give the plus four 95% confidence interval for the proportion of heterozygotes in the population of red-eyed fruit flies studied.