What is the units digit of the sum of all positive multiples of 6 smaller than 600?
If we look at the units digit of the first couple multiples of 6, we see
6, 2, 8, 4, 0, 6, ...
It repeats after the fifth term, and every set of those 5 numbers sums up to 20, which has a units digit of 0. That means that the multiples up to 600 also has a units digit of 0, as every set of five is equal to 0. We are looking for the sum of multiples smaller than 600, meaning it would go up to 594, but that does not matter as 600 has a units digit of 0. Therefore, the units digit is 0.