Number of copper (atomic weight 64) atoms present in a coin made up of bell metal is 
$3.0115 \times {10}^{22}$. The weight of the coin in grams is  ………..

Weight of  $6.023 \times {10}^{23}$ Cu atoms = 64 g 
Weight of  $3.0115 \times {10}^{22}$ Cu atoms = 3.2 g.
80 g of copper is present in 100 g of bell metal.
Therefore, weight of bell metal coin, in grams =  $3.2\times \frac{100}{80}=4$.