If the vehicle speed is increased by 20%, the time required to complete the journey will become 1÷( 1+20%) = 5/6 of the original time.

Available: the original time is 1÷( 1-5/6) = 6 hours;

After speeding up, the time required to complete the journey becomes 1- 1/9 = 8/9 of the original time, and the speed becomes 1÷(8/9) = 9/8 of the original speed.

Available: the original speed is 5 ÷ (9/8-1) = 40km/h;

Therefore, the distance between Party A and Party B is 40× 6 = 240km.