12 L× 5 times = 60 l.

100 km× 6 times = 600 km

The fuel consumption of a car driving 600km is 8.6 liters× 6 = 51.6 liters.

The amount of oil carried by the car after five round trips is 12 liters ×5=60 liters.

Because 5 1.6 liters is less than 60 liters.

So it is enough to add 12 liter of oil to the car.

Answer: the car plus 12 liter is enough for the car to go back and forth.

Thinking: It is known that the distance between A and B is 60km, and the car consumes 8.6 liters of fuel per 100km.

Because you can't use division, you can't find the fuel consumption without kilometers.

However, the fuel consumption of multiple distances of 100km can be calculated.

So 60km× 10 times =600km (because 600 is the least common multiple of 100 and 60, the multiplier must be a multiple of 2).

Therefore, it is more convenient to take 600 by multiplying 10).

And because 10 60km is five round trips.

Therefore, the amount of oil carried by the car is 12 liters ×5 times =60 liters.

Because cars consume 8.6 liters of fuel per 100km.

So the fuel consumption of six 100km (600km) is 8.6 liters ×6=5 1.6 liters.

Because the fuel load is 60 liters higher than the fuel consumption > 5 1.6 liters.

So 12 liter car is enough to go back and forth.