Answer:
Isolde alone will finish the box in 3 weeks.
Step-by-step explanation:
In terms of boxes per week, the rates are assumed to add up:
together = tristan + isolde
1/2 = 1/6 + isolde
3/6 - 1/6 = isolde = 2/6 = 1/3
Isolde alone eats 1/3 box per week, so a box will last 3 weeks.