Since you mention the word formula, I am assuming you want to use the spreadsheet, Excel. You indicate you understand the use of the TI calculator. Using Excel formulas, please do the following:
Load a blank Excel spreadsheet and select any cell to begin.
Click on the Insert Function icon next to the formula bar.
Select FV under the Financial category.
Click on the OK button.
Put data in each text box beginning at the top:
Place .05 in the Rate box. Press the Tab key. (The TI takes "5" here and converts it to a percentage internally. Excel needed the number formatted as a decimal fraction.
Place 20 in the Nper box for the number of periods (20 years as given in your problem)
Place 100 for the monthly deduction mentioned in your scenario.
Place 0 in the PV box for a zero beginning balance.
There is no need to put anything in the Type box which defaults to the end of month schedule you mention.
The answer displays in the lower right corner of the Function Argument dialog box as -3306.59541
Click on the OK button and your answer will appear in the original cell and the formula will appear in the formula box just to the right of the Insert Function icon.
This is a negative number because of Excel's convention here showing a reverse flow of the inputs as output. It is not a mathematical negative. Your answer is $3,306.60 rounded to the nearest cent. This balance was calculated in simple interest compounded yearly as you described with the contribution made at the end of each month.
