We have the balanced equation (without state symbols):
#Cu+2AgNO_3->2Ag+Cu(NO_3)_2#
We immediately see the ratio between #Cu# and #Ag# is #1:2#, so every mole of copper creates two moles of silver.
So, we got #20 \ "g"# of copper, and to convert that amount to moles, we divide by the molar mass.
Copper has a molar mass of #63.546 \ "g/mol"#. So here, there exist
#(20color(red)cancelcolor(black)"g")/(63.546color(red)cancelcolor(black)"g""/mol")~~0.31 \ "mol"#
Since the mole ratio is #1:2# between the desired reactants and products, we will produce #0.31*2=0.62 \ "mol"# of silver.
Silver has a molar mass of #107.8682 \ "g/mol"#. So here, we have a lump of silver that weighs:
#0.62color(red)cancelcolor(black)"mol"*(107.8682 \ "g")/(color(red)cancelcolor(black)"mol")#
#=66.88 \ "g"#
