FP1 General solution

I've done this question, but the answer I get is different to the mark scheme.

LET # = pi


It's jan 2011 AQA - Sin(4x-2#/3)=-0.5

I've used x = 2#n + principal value to give me x = #n/2 + 1/8# which is right

but then when using 2#n + # - principal value I got #n/2 + 11/24# and the right answer is #n/2 - 1#/24

could someone help?