I want to know how dofollow links help to increase page rank?
There is no such thing as “dofollow”, I think you’re getting confused with the rel=“nofollow” attribute having an opposite value which it doesn’t.
Dofollow links help to increase link juice which further help to increase page rank of the website.
As bluedreamer said, there is no such attribute as “dofollow”. Presumably the OP meant links which are not marked as “nofollow”, but as they’re no longer with us, there seems little point in pursuing this.
Thread closed.