Answer:
6.438 [litres].
Explanation:
1) the condition '5.50 liters are removed' means the coolant 12.5-5.5=7 [litres] with 13.4% solution;
2) the condition 'coolant containing 13.4% antifreeze' means 7*0.134=0.938 [litres] of pure antifreeze in the 7 litres coolant;
3) the condition '5.50 liters of pure antifreeze is then added' means 0.938+5.5 [litres];
4) finally, it is contained 0.938+5.5=6.438 [litres] of antifreeze will be contained in the final mixture.