Instead of putting 30, 36, 42 etc in the formulae explicitly, you can
calculate what it should be from the row that it is on. Here's an
example which works in a cell on row 21:

=(ROW()-20)*6+24

or, better still:

=ROW(A1)*6+24

(better because it is independent of the row you put it on)

This will return 30. If you copy it down it will return 36, 42 etc on
successive rows. So, just replace your first 30 with ROW(A1)*6+24 in
the formula in E21. Your formula might also become a bit easier to
read if you define a named range for Marks!$F$14:$DG$65 (eg call it
table), then your formula becomes:

=VLOOKUP($A$3,table,30,FALSE)

or with my suggestions above:

=VLOOKUP($A$3,table,ROW(A1)*6+24,FALSE)

Note that if you move this formula onto a different row (eg by
inserting rows above it), then it should still work.

Hope this helps.

Pete