Present Value of a Perpetuity

(Please Explain Step by Step)

A wealthy grandfather has one grandson and expects a granddaughter in two years.

He wants to invest now (at 8% per year) so that he can give $20,000 to his grandson at the end of this year and every year after.

He also wants to give $20,000 to his unborn granddaughter starting two years from now (when she is born) and every year thereafter.

How much does he need to invest now in order to provide the required funds for both kids?