How many silver (ag) atoms are in a silver bar that is 20.0 g? ?

To calculate this we first need to calculate the number of moles of Ag atoms:
moles = mass/RAM (Relative Atomic Mass) where the RAM of Ag = 108 g/mol.
Therefore moles(Ag) = 20.0g / 108g/mol = 0.185 mol
In one mole of Ag there are 6.022 x 10^23 atoms (Avogadro's constant), therefore the number of atoms in 20.0g of silver is:
(6.022 x 10^23)*0.185 = 1.11 x 10^23 atoms