First, balance the equation
Fe2O3 + CO ---> Fe + CO2
Fe2O3 + 3CO yields 2Fe + 3CO2 (Balanced)
Find the limiting reactant. The ratio of iron (iii) oxide to Carbon Monoxide should be 1:3.
You can only use 1/3 as much Fe2O3 as you can CO, as it is the limiting reactant and must follow the ratio.
11.5 g / 3 = 3.833333 g Fe2O3
The ratio of Fe2O3 the reactant to pure iron the product is 1:2, so just multiply by 2
3.833333 g Fe2O3 x 2 = 7.67 g Fe
This calculated amount is the theoretical yield, but in real life, there was some spillage. Only 6.10 g was produced.
6.10 g (actual yield) / 7.67 g (theoretical yield) = .7967 x 100 = 80.0 % yield
Hope I helped!