How do I figure out her round trip walk math problem?

Abby walks from town A to town B at a constant speed of 3 mph and back from town B to town A at a constant speed of 5mph. What is Abby’s average speed on the round trip walk? Please explain how you got this. Also What would be Abby’s average speed on the round trip if she walked one way at x mph and the other way at y mph?
 
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