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.
Personally I find that the more work is done on my hair and the more it will cost me, the more tips I will leave. A simple haircut might lead to 10-12% (e.g. for a $40 haircut I usually leave $5 extra). On the other hand when my hairdresser has spent several hours on color, highlights and styling, I do tip her better because I know that she deserves it.