How to determine the equivalent weight of carbon dioxide correctly?
Metallic Carbonate means $\ce{M^_x(CO3)_y}$ .

For example there are $\ce{Li^_2(CO3)}$, $\ce{Ca(CO3)}$, and $\ce{Al^_2(CO3)_3}$  

and $\ce{M^_x(CO3)_y -&gt; M^_xO_y + yCO2}$

$PV=nRT$

n= 0.0500 moles carbon dioxide evolved

mass of the evolved carbon dioxide is 0.0500 moles $\times$ 44.0 g/mol = 2.20 grams

mass of the remaining metal oxide is 4.215 g - 2.20 g = 2.015  grams. 

Of the 2.015 grams of the remaining metal oxide, 0.0500 moles $\times$ 16.00 g/mol = 0.800 grams are oxide, while the mass of metal is 1.215 grams.

There are 24.3 grams of metal per mole of carbon dioxide evolved.

Because the ratio x:y is unknown, it is unknown whether 1.215 grams corresponds to 0.05 moles, 0.1 moles or some other number of moles.  

However, since there is one mole of carbon dioxide evolved per carbonate ion, and carbonate has a charge of -2, the molar mass of metal per charge of the metal ion will be 24.3/2 = 12.15 grams.  This is what is meant by equivalent weight of the metal in the question.