N = 320 
n = 4

uzorak = c(1,1,2,3)
xi = c(2,2,5,10)
Mi = c(1.2,1.2,0.2,0.5)
M = 80

pi = Mi/M
pi

x_hh = sum(xi/pi)/(N*n)
x_hh

D_x_hh_ocena = sum((xi/(N*pi)-x_hh)^2)/(n*(n-1))
D_x_hh_ocena