RK Rajput places "Theoretical Questions" at the end of each section. Page 72 likely ends with Q.2.1 or Q.2.2.
He didn’t sleep that night. He didn’t build the impossible machine either. Instead, he borrowed a friend’s worn copy of Rajput the next morning, found page 72, and solved the problem with pencil and paper. Refrigeration And Air Conditioning Book By Rk Rajput Pdf 72