Thank you. May i check after getting n greater than 18, how did you get the table below it?

As the question has mentioned, the amount he paid back to his father is at the end of 2n months. Hence n> 18 -> smallest n is 19 i.e. 38 months.

However, you need to compute whether payment is completed by the odd month i.e. 37th month since the formula doesn't not show you the payment for those months.