To find the combined fuel economy after using all three mechanisms, we need to calculate the overall reduction in fuel wastage.
Let's assume the initial fuel wastage is 100 units.
Mechanism A reduces fuel wastage by 30%, which means it saves 30% of the initial fuel wastage, which is 30% of 100 units = 30 units.
After using Mechanism A, the remaining fuel wastage is 100 units - 30 units = 70 units.
Mechanism B reduces fuel wastage by 20%, which means it saves 20% of the remaining fuel wastage, which is 20% of 70 units = 14 units.
After using Mechanism B, the remaining fuel wastage is 70 units - 14 units = 56 units.
Mechanism C reduces fuel wastage by 10%, which means it saves 10% of the remaining fuel wastage, which is 10% of 56 units = 5.6 units.
After using Mechanism C, the remaining fuel wastage is 56 units - 5.6 units = 50.4 units.
The fuel economy is calculated as the percentage of fuel saved compared to the initial fuel wastage.
The overall fuel saved is 100 units - 50.4 units = 49.6 units.
The fuel economy is (fuel saved / initial fuel wastage) * 100%.
Therefore, the fuel economy after using all three mechanisms is (49.6 units / 100 units) * 100% = 49.6%.
Since the question asks for the fuel economy after using all three mechanisms combined, the correct answer is A. 60%.