1. The problem statement, all variables and given/known data Refer to the photos, question 21 and 22(the one with a helicopter) 2. Relevant equations Both answers are A 3. The attempt at a solution for question 21, if the counts to ten, there are 9 intervals. I divide 20 by 9 and divide it again by 2 and get 1.1s not 0.5s. for question 22. I think it is C. Because the first reflected pulse has travelled 2h and the second reflected pulse has travelled 4h. I think it is impossible for the answer to be A, the pulse should travel slower in water.