Does this overstay my 72 hours?

Hi I have a flight from USA landing in Beijing and I will stay for full 72 hours as permitted by the visa free transit time in Beijing. However, my flight out of Beijing to Sydney has a 15 hour layover in Guangzhou. Am I able to stay 72 hours in Beijing AND also have the 15 hour additional layover in Guangzhou or does the total number of hours in China (in both Beigin and Guangzhou) have to be 72 hours? Thank you for your help!