EXTRA 2 MARKS (HARD). A student sells batteries for mobile phones for $30 each while purchasing them for $10 each. She starts her business with $100 in cash and no inventory. On her first day of activity, she buys 7 batteries in the morning. At noon, she eats a sandwich for $15 (this as an operational expense). She ends the day with a cash balance of $75. What was her net income for that day?