In case the estimates were made with no real experience and are more best guesses use the triangular. But when there is already experience from similar previous projects or experts are doing the estimates you should use the beta distribution . Is there such information in the question about the "quality" of the estimates?

For L06.99 Self-Assessment for Module 06, Question #8, a scenario is given to figure out the expected activity duration using PERT. How do you know whether to choose Triangular Distribution or Beta Distribution in this case? I chose Triangular and got it wrong. I'd like to know why Beta is correct. Thanks.