A Maths Dictionary for Kids Quick Reference is a device friendly html version with definitions and detailed examples for over 950 math words and terms.
The statistics about the average cost of a haircut vary, but a few years ago American Salon, a publication for beauty industry professionals, reported the numbers as $21 for a cut in a salon with fewer than 6 chairs, and $44 for a salon with more than 13 chairs.
I'd say, haircut-wise, under the $100 (excessive to me at least) level, the cost reflects the time and detail spent on the haircut, therefore 20% makes sense. For instance, at my last job I charged $30-35 for a men's cut and $50-55 for a women's cut.