So the 4/5 liar could not respond like this? TLLLL LLLLT ..... .....
That is, he could never lie on 5 consequetive responses?
That makes it much easier.
He could. What is meant in the op is that for every set of five questions the 4/5 liar will lie four times (with 4 lies at the end possibly). if asked another set of five questions the liar will again lie four out of five times (with 4 lies at the beginning this time). So:
if asked 5 questions
T L L L L (possibly)
next 5 questions
L L L L T (possibly)
so T L L L L L L L L T = 8L / 10total which reduces to 4/5