Question

The weight of a small Starbucks coffee is a normally distributed random variable with a mean of 400 grams and a standard deviation of 20grams. Find the weight that corresponds to each event.

(Use Excel or Appendix C to calculate the z-value. Round your final answers to 2 decimal places.)
a. Highest 20 percent
b. Middle 60 percent to
c. Highest 80 percent
d. Lowest 15 percent.​

Answer 1

a. To find the weight corresponding to the highest 20 percent of coffees, we need to find the z-score that corresponds to the 0.8 cumulative probability.

Using a standard normal table or a calculator that can compute z-scores, we can find that the z-score corresponding to a cumulative probability of 0.8 is 0.8416.

Next, we use the z-score formula to find the weight:

Weight = Mean + (z-score * Standard Deviation)

Weight = 400 + (0.8416 * 20) = 448.32 g

So, the weight corresponding to the highest 20 percent of coffees is approximately 448.32 g.

b. To find the weight corresponding to the middle 60 percent of coffees, we need to find the z-scores that correspond to the cumulative probabilities of 0.2 and 0.8.

Using a standard normal table or a calculator that can compute z-scores, we can find that the z-score corresponding to a cumulative probability of 0.2 is -0.8416, and the z-score corresponding to a cumulative probability of 0.8 is 0.8416.

Next, we use the z-score formula to find the weights:

Weight Lower Bound = Mean + (z-score * Standard Deviation)

Weight Lower Bound = 400 + (-0.8416 * 20) = 351.68 g

Weight Upper Bound = Mean + (z-score * Standard Deviation)

Weight Upper Bound = 400 + (0.8416 * 20) = 448.32 g

So, the weight corresponding to the middle 60 percent of coffees ranges from approximately 351.68 g to approximately 448.32 g.

c. To find the weight corresponding to the highest 80 percent of coffees, we use the same process as in (a) to find the z-score that corresponds to the cumulative probability of 0.8, which is 0.8416.

Next, we use the z-score formula to find the weight:

Weight = Mean + (z-score * Standard Deviation)

Weight = 400 + (0.8416 * 20) = 448.32 g

So, the weight corresponding to the highest 80 percent of coffees is approximately 448.32 g.

d. To find the weight corresponding to the lowest 15 percent of coffees, we need to find the z-score that corresponds to the cumulative probability of 0.15.

Using a standard normal table or a calculator that can compute z-scores, we can find that the z-score corresponding to a cumulative probability of 0.15 is -0.9332.

Next, we use the z-score formula to find the weight:

Weight = Mean + (z-score * Standard Deviation)

Weight = 400 + (-0.9332 * 20) = 346.64 g

So, the weight corresponding to the lowest 15 percent of coffees is approximately 346.64 g.

Explanation: