1/( 1/7+ 1/9)=63/ 19=3.9375

Description: It took the goose seven days to fly from the South China Sea to the North China Sea and nine days to fly from the North China Sea to the South China Sea. It can be seen that the whole journey from the North Sea to the South China Sea is 7*9=63, the flight time 1/7, that is, 9, the flight time 1/9, that is, 7, and the total flight time is 7+9= 16. So 63/ 19=3.9375.