Cassandra read 25 pages of a 125-page book. What percent of the book did she read?

25 pages out of 125 means she has read $\frac{25}{125} = \frac{1}{5}$ of the book
As percent means "per hundred", we must convert this to hundred parts, i.e. $\frac{p}{100}$ As $\frac{1}{5} = \frac{20}{100}$,