a)
N = 35 + 87 + 89 + 59 + 17 = 287
μ = 30·35/287 + 40·87/287 + 50·89/287 + 60·59/287 + 70·17/287 = 13710/287 = 47.77
V = 30^2·35/287 + 40^2·87/287 + 50^2·89/287 + 60^2·59/287 + 70^2·17/287 - (13710/287)^2 = 9750200/82369 = 118.4
σ = √(9750200/82369) = 10.88