What is the sum of all the multiples of 3 in the 100 numbers 1-100?

What is the sum of all the multiples of 3 in the 100 numbers 1-100?

In 1-100, the multiples of 3 are 3, 6, 9 ,99
Form an arithmetic sequence, the first term is 3, the tolerance is 3, there are a total of (99 -- 3) / 3 + 1 = 33 terms
So the sum of the sequence is (3 + 99) x33 / 2 = 1683