First of all, let's assume that the number of days to lend money is X days.

Four comic books and four story books are lent every day, so in X days, the total number of comic books lent is 4x, and the total number of story books lent is 4x.

According to the requirements of the topic, the number of remaining comics should be three times that of story books, that is, the number of remaining comics is 3(4x).

Since there are 140 comic books and 100 story books in the library, we can list the equation:

140 - 4x = 3( 100 - 4x)

Next, we solve the value of x by solving this equation:

140 - 4x = 300 - 12x

Move the x term to one side and the constant term to the other side, and you get:

12x - 4x = 300 - 140

Simplified calculation:

8x = 160

x = 160 / 8

x = 20

So, after 20 days, there are three times as many comic books as story books.

I hope this calculation step is helpful to you!