Solution: Assuming that it costs X yuan to buy B zongzi, then A zongzi costs x- 100 yuan. If the unit price of B is Y yuan/piece, then the unit price of A is 120%y yuan/piece.

X- 100+x=700，x=400，x- 100=300。

300 ÷ (120% y)+400 ÷ y = 260: y = 2.5, then 120%y=3.

300÷3= 100 (piece), 400÷2.5= 160 (piece)

A: The unit prices of Party A and Party B are 3 yuan/Taiwan and 2.5 yuan/Taiwan respectively. Party A and Party B buy 100 and 160 respectively.