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Which situation is an example of an observational study?

Aday's production of 850 parts contains 50 defective parts. two parts are selected at random without r replacement. let the random variable x equal the number of defective parts in the sample. find the cumulative distribution function of x.

In a fifth grade class, 3/5 of the girls have brown hair. of the brown-haired girls, 3/4 of them have long hair. of the girls with long brown hair, 1/3 of them have green eyes. what fraction of the girls in the class have long brown hair?