Given:
interest rate = 2.76%
simple interest = 3,090
term = 15 years
Looking for the principal.
Simple Interest = Principal * interest rate * term
3,090 = Principal * .0276 * 15
3,090 = Principal * 0.414
3,090 / 0.414 = Principal
7,463.77 = Principal
Nate should invest 7,463.77 to meet his goal.