There are some salmon sushi, tuna sushi and shrimp sushi on a tray. The ratio of the number of salmon sushi to that of tuna sushi is 5: 3. The ratio of the number of tunas sushi to that of shrimp sushi is 2:3. (a) Find the ratio of the number of salmon sushi to that of tunas sushi to that of shrimp sushi. (b) There are 20 salmon sushi on the tray. Mary claims that the total number of sushi on the tray is 60. Do you agree? Explain your answer. (5 marks)