When 1+2+..........+n=n(n+1)/2, what is the sum of all the positive mu

Half the challenge might be wading through answers as formatted.

For sums of multiples of sequences, I do not use factoring or n(n-1)/2.

I used the general formula for summing [multiples of] an arithmetic sequence, then found the value that matched my answer.

Formula for sum of multiples of arithmetic sequences:

(Average)(# of terms), where

Average = \(\frac{FirstTerm + Last Term}{2}\) and

# of Terms: \(\frac{Last - First}{Increment}\) + 1

Average: \(\frac{6 + 714}{2}\) = 360

# of Terms: \(\frac{708}{6}\) = 118 + 1 = 119

Sum of multiples of 6, from 6 to 714 inclusive, is (360)(119)

Check answers (and toss out B and D immediately, they're huge, and C, which does not end in a zero).

Answer A is a match.
3*119*120 = (119)(360)

ANSWER A