How much is a good price for a teen to charge to mow lawns?

Say for a typical 1/3 acre home lot. Is it best to charge by the hour, or by the lawn? Since the answers may vary by region, please let me know what part of the county you live in (just general area, no specific details please!) I live in the midwest.

Answers:
I live in Texas. I would charge $20-$25 per lawn if it isn't going to take longer than an hour. If it is, I would charge $15 an hour. (I live in a small town, so pricing may differ from bigger cities in Texas)

Other answers:
I live in the Dallas area and they are getting about 40 to 50 dollars depending ion how much you want done.
I live in the Dallas area and they are getting about 40 to 50 dollars depending ion how much you want done.
I SAY YOU SHOULD PAY THEM $30 AN HOUR......................
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