To determine the opportunity cost of Sergei's best decision, we first need to identify which job he will choose based on the highest earnings.
The potential earnings are:
- Raking leaves: $50
- Making deliveries: $70
- Babysitting: $65
The highest earning option is making deliveries for $70.
The opportunity cost is the next best alternative that Sergei could have chosen, which in this case is babysitting for $65.
Therefore, the opportunity cost of the best decision (making deliveries for $70) is $65.