Random Variables

Given the probability model in the table below, what is the expected value of the random variable?
  X        50          20        5
P(X)     0.1         0.3      0.6

• 14
• 4.67
• 5
• 7.5

Is this a probability distribution?

• No, the sum of p(x) does not equal 1.

• Yes, all p(x) are between 0 and 1.

• No, all p(x) are not between 0 and 1.

• Yes, the sum p(x) is 1.

A small store keeps track of the number X of customers that make a purchase during the first hour that the store is open each day. Based on the records, X has the following probability distribution. What is the average number of customers that the owner can expect to make a purchase in the first hour of opening?

• 0.2

• 1.4

• 2.0

• 3.0

A marketing survey compiled data on the number of cars in households.  If X = the number of cars in a randomly selected household, and we omit the rare cases of more than 5 cars, then X has the following probability distribution: 
X           0          1          2          3           4           5    
P(X)   0.24    0.37    0.20    0.11    0.05     0.03
What is the probability that a randomly chosen household has at least two cars?

• 0.20
• 0.29
• 0.39
• 0.61

The mean weight of a bag checked in at Southwest Airlines is 36 pounds with a standard deviation of 3.5 pounds. If two random bags were checked, what is the mean and the standard deviation of the weights of the two bags?

• 36, 4.95

• 36, 7

• 72, 4.95

• 72, 7

The mean weight of a dog is 34 pounds with a standard deviation of 6.7 pounds. The mean weight of a cat is 12 pounds with a standard deviation of 3.1 pounds. If you have 2 dogs and 1 cat, what is the mean and standard deviation of their total weight?

• 80, 9.969

• 80, 16.5

• 46, 7.382

• 46, 16.5

Let the random variable X be the number of best friends each of a Grade 3 student. The probability distribution of X is shown on the left. What is the probability histogram of this distribution?

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The mean amount of time to mix the batter for a cake is 14 minutes with a standard deviation of 2.5 minutes. The mean amount of time to bake the cake is 35 minutes with a standard deviation of 6.2 minutes. What is the mean time to make and bake a cake? What is the standard deviation?

• mean = 49 min sd = 8.7 min

• mean = 49 min sd = 6.685 min

• mean = 21 min sd = 8.7 min

• mean = 21 min sd = 6.685 min

A coin is tossed and then a regular 6-sided die is rolled. What is the probability of the resulting outcome being

(tails, even)?

• 1/2

• 1/12

• 1/6

• 1/4

There are 3 red marbles, 4 blue marbles, 2 green marbles, and 6 clear marbles in a burlap bag. One marble is randomly pulled from the bag. What is the probability the marble was green?

• 1/2

• 1/7

• 2/15

• 2/17