Want to help your seventh-grader master math ... grade. At a “25% Off” sale, Marissa buys a skirt for $40.50. What was the original price of the skirt? Adding sales tax of 6%, what is the ...

perfectly add up to the number you began with. 6, for example, is perfect, because its factors — 3, 2, and 1 — all sum up to 6. 28 is perfect too: 14, 7, 4, 2, and 1 add up to 28.

Sponsor Message Weekend Edition "Math Guy" Keith Devlin tells the story of this arithmetic revolution in his new book, The Man of Numbers. Numerals 0 to 9 had been around in Hindu and Arabic ...