In a certain town, the time, X hours, for which people watch television in a week has a normal distribution with mean 15.8 hours and standard deviation 4.2 hours.
\(Find the value of k such that P(X < k) = 0.75.\)
Trees in the Redian forest are classified as tall, medium or short, according to their height. The heights can be modelled by a normal distribution with mean 40 m and standard deviation 12 m. Trees with a height of less than 25 m are classified as short.
(a) Find the probability that a randomly chosen tree is classified as short.
Of the trees that are classified as tall or medium, one third are tall and two thirds are medium.
(b) Show that the probability that a randomly chosen tree is classified as tall is 0.298, correct to 3 decimal places.
(c) Find the height above which trees are classified as tall.
The lengths of male snakes of this species also have a normal distribution. A scientist measures the lengths of a random sample of 200 male snakes of this species. He finds that 32 have lengths less than 45 cm and 17 have lengths more than 56 cm.
Find estimates for the mean and standard deviation of the lengths of male snakes of this species.
The weights of apples of a certain variety are normally distributed with mean 82 grams. 22% of these apples have a weight greater than 87 grams.
(a) Find the standard deviation of the weights of these apples.
(b) Find the probability that the weight of a randomly chosen apple of this variety differs from the mean weight by less than 4 grams.
The heights of students at the Mainland college are normally distributed with mean 148 cm and standard deviation 8 cm.
The probability that a Mainland student chosen at random has a height less than h cm is 0.67. Find the value of h.