Bell metal contains 80% copper. The mass of Bell metal which contains 1.5 × 10²⁰ atoms of copper is (Cu=64)

100gm of bell metal contains 80gm of 'Cu'

${\mathrm{n}}_{\mathrm{Cu}}=\frac{80}{64}=1.25\mathrm{moles}$

no. of atoms of 'Cu'=$1.25N=1.25\times 6.023\times {10}^{23}=7.53\times {10}^{23}$

100gm of bell metal contains 7.53$\times$10²³ atoms of Cu

The mass of Bell metal which contains 1.5 × 10²⁰ atoms of copper=$\frac{1.5\times {10}^{20}}{7.53\times {10}^{23}}\times 100=0.02\mathrm{g}=20\mathrm{mg}$