Find and compare the speeds of the boats in still water based on the given conditions. Boat A takes the same amount of time to travel 2x km upstream as it does to travel 3x km downstream. Boat B takes the same amount of time to travel 3y km upstream as it does to travel 5y km downstream. Boat C takes the same amount of time to travel 5z km upstream as it does to travel 7z km downstream. Quantity I: Boat A covers 144 km downstream and then returns to the starting point, completing the round trip in 20 hours. Find the speed of Boat A in still water. Quantity II: Boat B covers 150 km downstream and returns to the starting point, completing the round trip in 16 hours. Find the speed of Boat B in still water. Quantity III: Boat C covers 140 km downstream and returns to the starting point, completing the round trip in 12 hours. Find the speed of Boat C in still water.