I used this equation - 100 * (1 - (75.5/78.25)) = 3.514376...(Rounded to the tenths place you get 3.5)
Breakdown: You want to get your error to do this first you get your percent that was correct which is - (75.5/78.25)
You then subtract this by one because this will give you your margin of error as a decimal hence - (1 - (75.5/78.25))
Lastly, multiply this by 100 because you want to convert the decimal into a percent then round to the nearest tenth of a percent resulting in 3.5% error.
Let's say H is Howard and E is Erin and L is Lee. Since Lee downloaded 16 songs L=16. If Lee downloaded 3 more songs than Howard then H+3=16 so H=13. Is howard downloaded 6 more songs than Erin that means E+6=15. Finally E= 9