It takes seven days for wild ducks to fly from the South China Sea to the North Sea. It is known that the flying speed of wild ducks is one day 1/7 units.

Geese can fly one-third of the whole journey from the North Sea to the South China Sea in three days, which shows that the flying speed of geese in one day is 1/9 units.

When they meet, the sum of their distances is exactly the whole journey, that is, the unit is 1, so the meeting time = the sum of distances/speed.

That is,1÷ (1/7+1/9) = 63/16 days.

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