Too late for your assignment but on time to learn. Whenever you have a problem related to rates (such as the case of waves per second or calls per minute in a call center or fires per hour for a fire department and so on...) you need to apply Poisson Distribution. The formula is: $P(X=k)=\lambda^ke^{-\lambda}/k!$Lambda (λ) is the number of occurrences during the period of observation. In our case λ=(2 waves/second) x (5 seconds)=10 waves in 5 seconds as average rate. Your formula will be then: $P(X=k)=10^ke^{-10}/k!$ The probability of observing less than 15 waves is the probability of observing up to 14 waves (in other words observing 0,1,2,3....14 waves). This can be expressed as:$P(X < 15)=P(X \le 14)=\sum_{k=0}^{k=14}P(X=k)$ and the probability of observing 15 or more is$P(X \ge 15)=1-P(X<15)=1-P(X \le 14)$ All this can be put as: $P(X \ge 15)=1-\sum_{k=0}^{k=14}10^ke^{-10}/k!$ If you use Excel to calculate it, you get:$P(X \ge 15)=1-0.9165=0.0835$ and that is the same than 8.35%