Assume that an investor can invest the first $100,000 for a 13% return, an additional $100,000 yielding 10%, a further $100,000 earning 8% and the last $100,000 producing a 7% return. The cost of borrowing is 2% for the first $100,000, 4% for the next $100,000, 7.5% for an additional $100,000 and 10% to borrow yet another $100,000. What is the total amount which should be invested in this case? Why?