You can put this solution on YOUR website!
For this problem, we set up two variables, "t" is the rate (in boxes/week) that Tristan eats and "s" is the rate (boxes/week) that Isolde eats. We know that rate (boxes/week) times time (weeks) gives the number of boxes eaten.
So if "they share a box it last for two weeks" means that they're both contributing their rates (s & t) for two weeks (2) and in that time eating one (1) box, or:
The other equation says that "he eats a box of chocolates in 6 weeks" or:
which rearranges to:
Substituting into the first equation:
So she's her "eating rate" is 1/3 boxes/week, so she will finish a box of candy alone in 3 weeks.