② Party B: 4/ 10=2/5. When Party A and Party B draw a difficult problem, it is 2/5*2/5=4/25.
③ A failed to be drawn: 1-2/5=3/5, B: 2/5.
Rule: A didn't draw a puzzle, and when B did, 3/5*2/5=6/25.
④ A: 2/5，B: 2/5。 C: 4/ 10=2/5
Party A, Party B and Party C have drawn a difficult problem: 2/5*2/5*2/5=8/75.