Let's say there are 10 people.

10 * 30% = 3 people buy super

10 * 60% = 6 people buy unleaded

10 * 10% = 1 person buys diesel

6 * 20% = 1.2 people that buy unleaded will fill their tank

1.2 / 10 = .12 (12%) probability that the car leaving will with a full tank will have unleaded.

Does this follow with what you have for an answer?