Solution:
The fruit seller bought 8 kg of apples for ₹640
Cost price of 1kg apple = ₹640
8= ₹80
He sold the 8 kg of apples for ₹800
Selling price of 1kg apple = ₹800 8= ₹100
Profit = earnings - expense = ₹100 - ₹80 = ₹20
The fruit seller earns ₹20 per kg.