Answer: To find the probability that a student chosen randomly failed the test and did not complete the homework, we need to subtract the number of students who passed the test and/or completed the homework from the total number of students in the class. Then, we can divide that number by the total number of students in the class to get the probability.
To start, we can use the information given in the problem to create a Venn diagram:
```
Test
|------|------|
| | |
Fail | | |
| | |
--------|------|------|
| | |
| | |
Pass | | |
Assignment|------|------|
| | |
| | |
```
From the diagram, we can see that the number of students who failed the test and did not complete the homework is the number of students outside the intersection of the two circles. To find this number, we can add the number of students who passed the test but did not complete the homework to the number of students who completed the homework but did not pass the test, and then subtract the number of students who passed the test and completed the homework:
Number of students who failed the test and did not complete the homework = (Number of students who passed the test but did not complete the homework) + (Number of students who completed the homework but did not pass the test) - (Number of students who passed the test and completed the homework)
Number of students who failed the test and did not complete the homework = (15 - 13) + (18 - 13) - 13
Number of students who failed the test and did not complete the homework = 4
Therefore, there are 4 students who failed the test and did not complete the homework.
To find the probability of choosing one of these students at random, we can divide the number of students who meet both conditions by the total number of students:
Probability of choosing a student who failed the test and did not complete the homework = Number of students who failed the test and did not complete the homework / Total number of students
Probability of choosing a student who failed the test and did not complete the homework = 4 / 23
Probability of choosing a student who failed the test and did not complete the homework ≈ 0.17
Therefore, the probability that a student chosen randomly from the class failed the test and did not complete the homework is approximately 0.17, or 17% (rounded to two decimal places).