Since the question didn't specify how long she would have to save for, let's just assume that it's one year. There are 52 weeks in a year which is $520, and on her birthday she would have an extra $100, so every year she saves $620 in total.
The 28% rule states that you should spend 28% or less of your monthly gross income on your mortgage payment (e.g. principal, interest, taxes and insurance). To determine how much you can afford using this rule, multiply your monthly gross income by 28%
