The exercise below asks questions about the sum or difference of two random
quantities. To answer each question, you should first determine the mean and standard
deviation of the distribution of the sum or difference.
- Evaluate the standard deviation of the difference or sum using the green template
on the top right.
- Find the mean of the difference or sum (using mental arithmetic!)
- Translate the difference or sum (0.0 in the initial question) into a z-score
with the second green template.
- Drag over the standard normal curve to find the probability of obtaining a
lower z-score.
- Type your answer to the question (which is based on the probability that you read from
the normal curve) into the text-edit box on the left, then click Check.
Repeat with different questions until you are confident with the method.