If I got this figured right, and say it right, see what answer you get.
On a planet far far away, there are twenty couples of exact same age to the minute. (That's just the way things happen on this planet OK!!!)
When they turn twenty years old all of them at the same time pop out a set of twins. One boy and one girl.
When these children turn 19 years old they get married. When they turn twenty, all of them at the same moment, they have a set of twins, one boy one girl. So is the way of this species.
So it is repeated with all offspring
This is all the kids each couple have.
Also on this planet these people (or creatures if you wish) die right when they turn 60 years old.
The question is, how many will populate this planet in 100 years?
How many in two hundred years.
And how many in three hundred years.
Hope I got that all correct.