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Looking for an ROI formula, brain is broken today
The monthly repayments of the initial $ 300,000 loan can be calculated using this formula:  source: Finance Formulas The monthly payment is It is not readily apparent how the formula works, but it is derived by induction from this summation, in which the sum of the discounted future payments are set equal to the present value of the loan:  For the second part of the question, reinvestments are stopped after 9 months, after four investments of $ 26,374.77 * 3 = $ 79,124.31.  And presumably each loan is repaid in 3 years, since 45 - 9 = 36 months. Calculating the repayments for these loans: The total returned for all four loans is: