Question

Sally gets a cup of coffee and a muffin every day for breakfast from one of the many coffee shops in her neighborhood. She picks a coffee shop each morning at random and independently of previous days. The average price of a cup of coffee is $1.40 with a standard deviation of 30¢ ($0.30), the average price of a muffin is $2.50 with a standard deviation of 15¢, and the two prices are independent of each other.

(a) What is the mean and standard deviation of the amount she spends on breakfast daily?
(b) What is the mean and standard deviation of the amount she spends on breakfast weekly (7 days)?

Answer 1

A. Mean of the amount Sally spends on breakfast daily = $ 3.90

Standard deviation of the amount Sally spends on breakfast daily = $ 0.34 (rounding to the next cent)

B. Mean of the amount Sally spends on breakfast weekly = $ 27.30

Standard deviation of the amount Sally spends on breakfast weekly = $ 0.89 (rounding to the next cent)

Explanation:

1. Let's review the information given to us to answer the question correctly:

Average price of a cup of coffee = $ 1.40

Standard deviation = $0.30

Average price of a muffin = $ 2.50

Standard deviation = $ 0.15

The two prices are independent of each other

2. What is the mean and standard deviation of the amount Sally spends on breakfast daily?

Let's recall that:

1. The mean of the sum of two independent variables is the sum of their means.

2. The variance of the sum of two independent variables is equal to the sum of their variances.

Therefore,

Mean of the amount Sally spends on breakfast daily = Average price of a cup of coffee + Average price of a muffin

Replacing with the values we know:

Mean of the amount Sally spends on breakfast daily = 1.40 + 2.50

Mean of the amount Sally spends on breakfast daily = $ 3.90

Variance of the amount Sally spends on breakfast daily = Variance of the price of a cup of coffee + Variance of the price of a muffin

Replacing with the values we know:

Variance of the amount Sally spends on breakfast daily = 0.30² + 0.15²

Variance of the amount Sally spends on breakfast daily = 0.1125

Standard deviation of the amount Sally spends on breakfast daily = √0.1125

Standard deviation of the amount Sally spends on breakfast daily = $ 0.34 (rounding to the next cent)

3. What is the mean and standard deviation of the amount she spends on breakfast weekly (7 days)?

Mean of the amount Sally spends on breakfast weekly = 7 * Mean of the amount Sally spends on breakfast daily

Mean of the amount Sally spends on breakfast weekly = 7 * 3.90

Mean of the amount Sally spends on breakfast weekly = $ 27.30

Variance of the amount Sally spends on breakfast weekly = 7 * Variance of the amount Sally spends on breakfast daily

Variance of the amount Sally spends on breakfast weekly = 7 * 0.1125

Variance of the amount Sally spends on breakfast weekly = 0.7875

Standard deviation of the amount Sally spends on breakfast weekly = √0.7875

Standard deviation of the amount Sally spends on breakfast weekly = $ 0.89 (rounding to the next cent)