Given the marks 15, 16, 17, 18, 19, 20, and assuming x=3 from a previous problem, find the probability that a randomly selected student scored a composite mark.

A composite number is a positive integer that has at least one divisor other than 1 and itself.
The marks are 15, 16, 17, 18, 19, 20.

• 15 is composite ($3 \times 5$).
• 16 is composite ($4 \times 4$).
• 17 is prime.
• 18 is composite ($2 \times 9$).
• 19 is prime.
• 20 is composite ($4 \times 5$). The composite marks are 15, 16, 18, and 20.

From the previous problem, we found $x=3$. The frequencies are:

• Mark 15: Frequency = 6
• Mark 16: Frequency = $x^2-1 = (3)^2-1 = 9-1 = 8$
• Mark 17: Frequency = $3x = 3(3) = 9$
• Mark 18: Frequency = 10
• Mark 19: Frequency = $3x-2 = 3(3)-2 = 9-2 = 7$
• Mark 20: Frequency = 5

The frequencies for the composite marks are:

• Frequency for mark 15 = 6
• Frequency for mark 16 = 8
• Frequency for mark 18 = 10
• Frequency for mark 20 = 5

Total number of students with composite marks = $6 + 8 + 10 + 5 = 29$.

The total number of students in the class is 45 (from the previous problem).
The probability that a randomly selected student scored a composite mark is the ratio of the number of students with composite marks to the total number of students.

The probability that the mark scored by the student is composite is $\boxed*{\frac{29}{45}}$.