If we do not go bankrupt in the middle scenario, we pay the debt payment with 2/3 probability. If we go bankrupt in the middle but not the best, we pay the debt payment with probability 1/3
To understand whether the best scenario will be bankruptcy, we ask ourselves whether there is an amount that we can promise the bank and not go bankrupt, while the bank has a positive NPV; or, equivalently, the expected discounted cash flow received by the bank exceeds 100. Therefore, we check the highest possible debt payment that does not lead to bankruptcy in the best scenario. Since the best scenario provides 400 in cash flows, 400 is the highest debt payment that does not lead to bankruptcy. 400 debt payment in a 100 loan means interest rate is 300%