V = l**3= 3**3 =27 cm3 = 27*10exp(-6) m3
dens = m/V m= dens*v=27**10exp(-6)*7800=0,2106 kg =210,6 g
56 g Fe..........................................6,022x10exp(23) atomi
210,6 g Fe.............................................x
x = 22,647xx10exp(23) atomi