With 6a, how do you calculate the greater percentage of students who prefer juice?

The method is
(the students prefer juice/the total number of students)*100%

for grade7:
[(11+13+20)/(12+19+11+13+20+5)]*100%
=(44/80)*100%
=55%
For grade8:
[(13+27+4)/(20+12+13+27+4+8)]*100%
=(44/84)*100%
=52.38%
55%>52.38%
Therefore the answer is grade 7