Lina fills up her car with petrol every week when it is half full. Since Lina first started driving the car they have always filled it up (from half full) with premium unleaded petrol. However, after some advice from their mechanic, they decide that from now on they will fill the car up with ultra premium unleaded petrol. The first time Lina fills their car up (from half full) with ultra premium unleaded petrol, the concentration of ultra premium unleaded petrol in the petrol tank is 12 ultra per litre of petrol. After Lina’s 12th top up with ultra premium unleaded, what is the concentration of ultra premium unleaded petrol in the petrol tank?