Looking for a pattern

Dear children,

For those of you who need help with the last MCQ question in the revision paper, here is the hint for you which I've also left on the whiteboard just now.

You will find this question challenging as it is different from the usual questions which have the numbers in running order. Instead it goes zig zag.

So, if you were to write out 2 rows together, you will have:

A B C D E F F E D C   B  A  A  B   C  D  E   F  F ...
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ...

Remember the PTS we've learnt recently on pattern recognition, try to find the repeating group, and you will be able to solve the question.

Try it again! The question isn't so hard after all right? ^^

Regards,
Miss Chan