The opportunity cost for interest is how much interest she would have earned if she had not invested in her current business. The amount that she actually invested in the current business was $200,000 - this is the amount that had been invested in the business in the form of the capital assets of the business.
If she were to have closed this business, she would have had to invest $50,000 in the other business. So, she would have had only $150,000 of 'cash' to invest at 5%. This is the $7,500 that is her opportunity cost of interest.
Does this help?