我希望在（grades.txt）下面返回哈希值（每个键）的平均值
Melotti, Suzanne: 100 100 95 95 92 87
Wayne, Bruce: 85 85 85 75 75 75
Stark, Tony: 92 92 75 79 91 87
Odinson, Thor: 23 12 10 42 50 64
Kane, Kathy: 100 100 100 100 95 95
Rogers, Steven: 92 91 91 90 87 84
Murdock, Matthew: 100 100 100 99 99 98
VonDoom, Victor: 75 75 72 73 74 80
Queen, Olvider: 92 83 74 65 100 66
Hall, Carter: 23 12 10 42 50 …