Answer: -
2 x 10 ²⁴ atoms of Ag rounded off to 1 significant figures.
Explanation: -
Mass of Cu = 100 g
Molar mass of Cu = 63.55 g / mol
Number of moles of Cu = 100 g / (63.55 g/mol)
= 1.574 mol of Cu
The balanced chemical equation for the reaction is
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2 Ag
From the balanced chemical equation we see
1 mol of Cu gives 2 moles of Ag
1.574 mol of Cu gives 
= 3.148 mol of Ag
1 mole of Ag has 6.02 x 10 ²³ atoms of Ag according to Avogadro's Law.
3.148 mol of Ag has 
= 18.95 x 10²³ atoms of Ag
= 2 x 10 ²⁴ atoms of Ag rounded off to 1 significant figures.