Question 558680
Sum = 3+9+15+...+93+99


Use the "Gauss" method where you list the addends in reverse order:


Sum = 3+9+15+...+93+99
Sum = 99+93+...+9+3


2*Sum = 102+102+... = 102*17 (102 occurs 17 times)


Sum = 51*17 = 867


Update: There isn't really much of an "algebraic" solution since we know the numbers we wish to add are the odd multiples of 3 (3, 9, 15, ..., 99) and adding them doesn't require much algebra. Even with an "algebraic" solution you should still obtain the same answer.