Question 2
Châu has four cards bearing four distinct natural numbers greater than 0. In round 1 she selects two cards whose average is 30. In round 2 she selects three cards whose average is 31. In round 3 she selects all four cards whose average is 32. How many different sets of four numbers satisfy these conditions? For example, 1,3,5,7 and 1,5,7,3 are the same set. Explain your reasoning.
