K?

E?

I have the answer it is $E

Looks like it was a lucky guess prompted by the apparent relative positioning of B, C, D! What about it, guys?

The letter after E would be X

Alas, it is more a tribute to memory than to puzzle-solving ability. I just had a flash of inspiration and remembered my maths teacher from 1978 who had a fascination for this kind of puzzle. He had set us a very similar teaser that took us weeks to crack (and with more than one or two clues!).

Is it time to give a clue on this one? I must say, the logic is one of the more obcure relationships.

A clue of sorts:

In solving letter sequences, there are usually 3 basic questions to ask:

1. Is there a mathematical progression? (eg, move 4 letters through the alphabet, then back two etc)
2. Is there a sequence based on initial letters of something occurring in general life, not mathematics? (MTWTFSS, OTTFFSS, JFMAMJJ)
3. Is there an intrinsic quality to the letters themselves that has nothing to do with the above?

OK, the first of the examples refers to Monday, Tuesday, Wednesday etc. I.e., these examples are not mathematical, but rely on lateral thought to make a conection with a sequence that is "familiar" in its normally occurring form. These 3 examples are the ones you would see most often, but sometimes you may get something really obscure like HHLBBC... (the periodic table of the elements: Hydrogen, Helium etc)

I suggest you focus on the 3rd category!

Another sequnce, this time a simpler one!

Where should the number 10 be placed to continue the sequence?

8   5   4   9   1   7   6   3   2

mathsyperson is correct, can someone other than him tell why 10 should be placed there?