so from 75 to 80 and 75 is 100%
80-75=5
5 is x% of 75
x%=percent markup
5=x% times 75
% means parts out of 100 so
x%=x/100
5=x/100 times 75
5=75x/100
75x/100=75/100 times x
75/100=3/4
5=3/4x
multiply both sides by 4/3
20/3=x
6.6666%
the percent mark up is 6.6666%
