total at first → 750
"He sold 50% of the cookies and" remaining → (100 - 50)% x 750 = 375
"and gave 2/3 of the remaining cookies to his neighbour. "
left (fraction) → 1 - 2/3 = 1/3
left → 1/3 x remaining = 1/3 x 375 = 125
"He then ate some cookies and had 80 cookies left."
ate → 125 - 80 = 45
ate % → 45 / 750 x 100% = 6%