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.

By using arithmetic progression (AP) series logic formula

a,a+d,a+2d,a+3d,……….

In this question, a =27

d – difference between the first and second number

d= 37- 27=10

We find out the fourth number,

a +3d = 27 + 3(10)

a + 3d = 27+ 30

a + 3d = 57

So 57 is the next term in the series.