In a library one of the student librarians we working on re-shelving some books and accidently spilled a bunch on the ground, the types of books are given in the following table:

	History	Math
Hardback	100	50
Paperback	200	20
Total	300	70

We want to know if we randomly grab a book off the pile what is the following probability

\[P(\text{paperback}|\text{math})\]

This probability is calculated by realizing that since we know that we pulled a math book then then we have restricted ourselves to only math books, that is we have the following:

\[P(\text{paperback}|\text{math})=\frac{\text{number of paperback math books}}{\text{number of math books}}=\frac{20}{70}\]