For CO2 gas PV=(wm)RT
700760×13361000=w44×0.0821×300
∴wCO2=2.20g
Suppose metal carbonate is M2(CO3)n and let atomic mass of metal be a, then
M2(CO3)n→M2On+nCO2
∵(2a+60n)gM2(CO3)n=44ngCO2
∴4.215gM2(CO3)n=44n×4.2152a+60ngCO2
∴44n×4.2152a+60n=2.20∴an=12.15
Also, equivalent mass of metal = a/n = 12.15
So, nearest integer is 12.