Answer: 15/2 or 7.5 weeks.
If t represents the number of weeks that pass, then the situation can be represented as
3/4 - (1/10)t = 0
because the initial amount is 3/4 and it is decreasing by 1/10 per week, and we are trying to find when the bag will be empty (0).
Solving this, (1/10)t=3/4 -> t=15/2.