Final answer:
To make a 21% oil-gas mixture, you need to mix oil and gas in the correct proportions. None of the given options matches the correct mixture.
Step-by-step explanation:
To make a 21% oil-gas mixture, we need to mix oil and gas in the correct proportions. Let's assume that x liters of the 80% oil-gas mixture and y liters of the 20% oil-gas mixture are used.
The total amount of oil in the 35 liters of the 21% mixture is 21% of 35, which is 7.35 liters. Since the 80% oil-gas mixture contains 80% oil, the amount of oil in x liters of this mixture is 0.8x liters. Similarly, the amount of oil in y liters of the 20% oil-gas mixture is 0.2y liters.
Setting up an equation, we have: 0.8x + 0.2y = 7.35
Since we have two unknowns x and y, we need another equation to solve this system. However, none of the given options match the correct mixture. Therefore, none of the options is correct.