f = amount of fliers expected to be mailed in 1 day.
a)
so, she has 5 days to mail those fliers, so
day 1........... 1f mailed out
day 2........... 2f mailed out
day 3........... 3f mailed out
day 4........... 4f mailed out
day 5........... 5f mailed out
so she'll be mailing out a total of 5f fliers, we know whatever "f" is, that total must be at least 850, now, it could be more, no less, could also be 850 exactly, but could also be more, 5f ⩾ 850.
b)