This is from p.120 of Ta-Pei Cheng's Relativity, Gravitation and Cosmology. I cannot understand the way the book calculated the age of the empty universe. The velocity $v$ of expansion is proportional to the distance $d$. That means the expansion velocity has not been constant. But why does the book conclude the age of universe from simply dividing the distance $d$ from the velocity of the current moment? Is there something that I am missing? I need help.