1) 19
1+2+3.../10 = whole number. We can first check 9, where (1+9)+(2+8)... = 45 so that doesn't work. The next number that does work is 19.
2) A
3+6+9+12+15 = 45, 50-45 = 5.Since 15 has groups of 5, 18 has groups of 3, and A falls in as the 5th letter
3) 12345678987654321 (actually)
I did some experiments with smaller number (99*99, 999*999), and you get a pattern of 12321 and 1234321, so with 9, 9's on each side, we get... that number.
4) 306
0 is also an integer, and since we have an even number of terms, we can start from 17, and count back to -18 (where 17+(-17) cancels out). We get that -18 * -17 = 306.
5) 2
we can take out the 1+2 temporarily to get 3+4+5... +327 = (3+327)+(4+326)... (and 327-3 is even so they match up).
Putting back in the 1+2, and adding 2, we get 5.