1, let n days later, b is twice as much as a.

(40+4n)×2=62+9n

The solution is n= 18.

So after 18 days, b is twice as much as a.

2. Suppose the price of a badminton and a table tennis is X yuan and Y yuan respectively.

6x+4y=3 1.4 ①

10x+8y=57 ②

The solution is x=2.9 y=3.5.

So the unit prices of badminton and table tennis are 2.9 yuan and 3.5 yuan respectively.

3. Let the number of pieces in pile A be n and the number of pieces in pile B be n- 100.

n+50=5(n- 100-50)

The solution is n=200.

Therefore, Party A and Party B have 200 pieces and 100 pieces respectively.