What is the units digit of the sum of all positive multiples of 6 smaller than \(600\)?
Sum==[F + L] / 2 x N, where F==First term, L==Last term, N==Number of terms.
The smallest multiple of 6 ==6. The largest multiple of 6 under 600==594
[594 - 6] / 6 + 1 ==98 + 1 ==99 terms.
Sum==[6 + 594] / 2 x 99 ==29,700. So, the units digit==0.
