Question 27313
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:
{{{(s+t)(2)=1}}}
The other equation says that "he eats a box of chocolates in 6 weeks" or:
{{{t*6=1}}} which rearranges to:
{{{t=1/6}}}
Substituting into the first equation:
{{{(s+1/6)(2)=1}}}
{{{2s+1/3=1}}}
{{{2s=2/3}}}
{{{s=1/3}}}
So she's her "eating rate" is 1/3 boxes/week, so she will finish a box of candy alone in 3 weeks.