White, black, white, black, black. Where are the sixth and tenth white balls?

White, black, white, black, black. Where are the sixth and tenth white balls?

The first white is 1
The second is 1 + 2 = 3
The third is 1 + 2 + 3 = 6
So the sixth is 1 + 2 + 3 + 4 + 5 + 6 = 21
The tenth is 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55