As settlement of perpetual contracts is in BTC, rather than being settled in USD, the Average Entry Price is calculated as follows:
Average Entry Price = Total number of contracts / Total contract value in BTC
Total contract value in BTC = [(Quantity A / Price A) + (Quantity B / Price B) + (Quantity C / Price C) ...]
A trader bought 80 contracts at 7000 USD and another 20 contracts at 7500 USD.
Total contract value in BTC = (80/7000) + (20/7500) = 0.014095
Average Entry Price = 100/0.014095= 7094.71 USD