Final answer:
The boxer must lose 1.8 kg in the final week to compete as a flyweight.
Step-by-step explanation:
To find the number of kilograms the boxer must lose in the final week,
we need to calculate the total weight loss in three weeks and subtract it from the total weight loss required in a month.
The boxer lost a total of 55.5 kg - 53.8 kg = 1.7 kg in three weeks.
In a month, the boxer needs to lose 3 1/2 kg = 3.5 kg.
Since three weeks have already passed, there is only one week left for the boxer to achieve the total weight loss required.
The boxer must lose 3.5 kg - 1.7 kg = 1.8 kg in the final week to be able to compete as a flyweight.