Since she spends $20 of the $125 she makes a month, she really ends up with $105 a month.
Write an equation using 'm' to represents months. It will be the money earned a month times the number of months, and this will equal how much she needs ($800)
105m=800
To find how many months, divide each side by 105
m equals about 7.62
Since you aren't going to have just .62 of a month, you need to round. Since it is above .5, you round up to 8 months.