Answer : 55

The series “1+2+3+4+5+6+7+8+9+10” can be considered as the sum of the first 10 Natural numbers.

The generalized formula to find the sum of first N natural numbers is

(N * (N + 1) ) /2

For the series given, the value of N is 10

So substituting value of N in the generalized formula

( 10 * (10 + 1) ) / 2

= (10 * 11 ) / 2

= 110/2

Correct answer = 55

Answer : Joel

Explanation:
If we consider the given numbers 6, 4, 9, 10, 11 to be the month number then we can easily find the killer.

6 – June – J
4 – April – A
9 – September – S
10 – October – O
11 – November – N
Hence the answer is JOEL.