Answer: $10,000
Explanation: True profits are the profits which takes into consideration the opportunity cost of choosing an alternative. Opportunity cost refers to the loss of profit incurred by a firm or an individual while choosing best alternative over the second best alternative.
In the given case, the individual could have earned $30,000 from the job so it is his opportunity cost that must be deducted from the revenues to ascertain the profit. That is :-
Profit = revenue - materials - opportunity cost
= $100,000 - $60,000 - $30,000
= $10,000