Answer:
7 hours
Step-by-step explanation:
The formular of percentile is given as;
n = (P / 100) * N
where;
n = ordinal rank
P = Percentile 
N = Number of values
The grades are given as;
5.5,  7,  6,  7, 6.5,  6,  9
, 6.5,  6.5
, 6.5
Arranging in increasing order;
5.5, 6, 6, 6.5, 6.5, 6.5, 6.5, 7, 7, 9
n = (81/100) * 10 = 8.1 ≈ 8
The 81st percentile is the 8th term.
The 8th term = 7