Physics help...sort of

Because Earth’s rotation is gradually slowing, the length of each day increases: The day
at the end of the 1.0 century is 1.0 ms longer than the day at the start of the century. In 20
centuries, what is the total of the daily increases in time (that is, the sum of the gain on the first
day, the gain on the second day)

From the PDF posted. There is that parenthesis which makes it pretty clear what it wants.

There is more then one science textbook in the world. The one i copied form is form mine. I'd gladly send you a pic if you wanted.