100% of solution is 70%pure
100g of solution of  MgCo_{3} contains 70g of  MgCo_{3}
so 60g of solution contains 70/100*60=42g
now 1 mole of  MgCo_{3} have 82g of  MgCo_{3} (molar mass of MgCo_{3} )
but we have 42/84=1/2=0.5 moles of it
also 82g of  MgCo_{3} contain only 44g of  Co_{2}
so 42 g of  MgCo_{3} contans only 22g of  Co_{2}
and 44g of  Co_{2}
has volume of 22.4 L at STP
so 22g of  Co_{2}
has volume of 11.2 L or
1120  cm^{2}