大家好，想请问一下1990-2050年，80岁以上年龄组的mean相加，用R语言要怎么做？
Age Year Mean
65 to 69 1990 12789962.48
70 to 74 1990 9379852.849
75 to 79 1990 6002410.44
80 to 84 1990 3161748.967
90 to 94 1990 227398.9816
85 to 89 1990 1123431.477
95 plus 1990 40112.93868
Under 5 1990 246288.0595
5 to 9 1990 1714892.89
10 to 14 1990 4555664.807
15 to 19 1990 9251368.059
20 to 24 1990 12070260.77
25 to 29 1990 12956600.91
30 to 34 1990 13732213.59
35 to 39 1990 18198647.03
40 to 44 1990 16433599.86
45 to 49 1990 15257949.08
50 to 54 1990 16468251.93
55 to 59 1990 16876521.79
60 to 64 1990 15208278.23
