so she is essentially saving 250+n*75, where n is the number of weeks.
after 8 weeks she will have 250+8*75, (you need to substitute n for the number of weeks, which is 8)
which is
250+600=850
how many more does she need for a car?
she is still missing 2000-850, that is 1150. divide it by 75: it's a little more than 15: so she actually needs to save for 16 weeks after she's been saving for 8, that's a total of 24 weeks.
so in total it will take her 24 weeks (and she will have a little extra).
