Call the number of people she invites i. If she invites i, then 0.4i will not attend, meaning that i - 0.4i = 0.6i will attend. The number who will attend is x, so:
0.6i = x
(3/5)i = x
i = (5/3)x

We're looking for the number of people she should invite, i, which is equal to (5/3)x, choice (D).