To help Alexandria Pappas get a B in math by averaging 80 on five tests, we can determine the lowest possible score on the last test by following these steps:

1. Step 1: Add the first four test scores. The first four scores are:
   84, 73, 83, and 78.

   $84 + 73 + 83 + 78 = 318$

2. Step 2: Calculate the total needed to average 80 over five tests.
   Since she needs an average of 80 across five tests, the total score needed is:

   $80 \times 5 = 400$

3. Step 3: Subtract the total of the first four tests from the required total.
   The score required on the last test is:

   $400 - 318 = 82$

So, the lowest score she can get on the last test and still get a B is 82.

Let me know if you would like more details or have any questions!

Here are some follow-up questions:

1. How would the result change if Alexandria needed a higher average, such as 85?
2. What is the highest possible score she could have earned on one of the earlier tests while still achieving a B?
3. How would the lowest score change if she had scored less than 73 on one of her tests?
4. Can you think of a formula that would help calculate the needed score given any set of test results?
5. How can the average be recalculated if a test score is dropped?

Tip: To find the required score for an average, remember that multiplying the desired average by the number of items (tests) gives you the target sum.