Metal carbonates undergo decomposition under heating to give the metal oxide and carbon dioxide, i.e.:
#CuCO_3(s) + Delta rarrCuO(s) + CO_2(g)uarr#
We start with #(31*g)/(123.57*g*mol^-1)# #=# #0.25# #mol#.
Copper oxide, #CuO#, has a formula mass of #79.6# #g*mol^-1#, of which #0.25# #mol# represents a #20# #g# mass. Here we use the reaction stoichiometry above: one equiv of copper salt gives one equiv of copper(II) oxide.
Note that copper carbonate is as far as anyone knows, #Cu_2(OH)_2CO_3#. Given the context of the question, an identity of #CuCO_3# is assumed. Typically, at A level, a student would be asked to estimate the volume of carbon dioxide gas evolved under standard conditions. This #CO_2# would be the same molar quantity, but of course would have a different mass and volume.
