Answer:
There are different ways to approach this problem, but one possible method is:
Define the event A as "the student owns a trampoline".
Find the probability of A: P(A) = 21/23, because 21 out of 23 students said they owned a trampoline.
Find the probability of the complement of A, which is "the student does not own a trampoline". We can denote this event as A', and it is equivalent to "the student is one of the 2 students who did not say they owned a trampoline". Therefore, P(A') = 2/23.
Check that P(A) + P(A') = 1, because the student either owns a trampoline or does not.
Answer the question: the probability that the student does not own a trampoline is P(A') = 2/23, or approximately 0.087 or 8.7% (rounded to one decimal place).
Therefore, the answer is: the probability that the student does not own a trampoline is 2/23, or approximately 0.087 or 8.7%.