Math (Probability)
posted by Dan .
When Lino submitted an essay for his English class, it was 9 pages long. Lino's essay had errors on pages 2 and 7. If the teacher chose 3 pages uniformly at random to read, then the probability that she didn't read any of the pages with errors can be expressed as a/b where a and b are coprime positive integers. What is the value of a+b?

17