Answer:
About 21 weeks
Step-by-step explination:
Her bike is 775 dollars. Lets start there.
She will deposit 40 dollars every Friday, or, every week.
With this logic, we would only need 20 weeks. I found this by doing 775/40.
BUT
We have one more complication. Andrea owes 45 dollars to her sister. It is important to note that this is a ONE TIME payback. This is NOT weekly.
Here is how we will do it:
Andrea needs 775(bike)+45(sister)=820 dollars in debt
820/40 a week=20.5 weeks.
Since .5 of a week is 3.5, and I don't think .5 of a day applies here, we will round up and say that andrea will need roughly 21 weeks to earn all the money she needs.
Final answer: 21 weeks