Actually Ryan's

(150 + 1)(150 / 2) = 11 325

150 / 3 = 50
// 50 multiples of 3
(50 + 1)(50 / 2) * 3 = 3 825

150 / 7 = 21,4285714286
// 21 multiples of 7
(21 + 1)(21 / 2) * 7 = 1 617

150 / 21 = 7,1428571429
// 7 multiples of 21
// (multiples of both 3 and 7)
(7 + 1)(7 / 2) * 21 = 588

11325 - 3825 - 1617 + 588 = 6 471