What is the monthly installment that will discharge a debt of $2455 due after one year at a 5% p.a. simple interest?

The monthly payments can be calculated using the Excel spreadsheet PMT formula. You can use the = sign in a cell with PMT(interest rate, number of payments, amount of the loan due). For this it will be =PMT(0.0042, 12, 2455,0). The 0.0042 is calculated from the interest of 5% divided by 12 months, divided by 100 to turn it into a decimal and then it is rounded to 42% or .0042. 12 are the number of payments, 2455 is the debt, and 0 is the future value when it is paid off. That is one way to calculate. The article outlines other methods as well.