I THINK there is a typo. Instead of "for problems each day", i think it's "four problems each day."
Anyway, we need to find how many days it takes Jeremy to complete the paper.
If he completes 8 problems on the first day and 4 on the rest of the days, after the first day he'll have 28 - 8 = 20 more problems.
And since he does 4 every day with 20 problems: 20 / 4 = 5 days.