At brian’s bookstore, 0.3 of the shelves hold
mysteries, 25% of the shelves hold travel books, and 7/20 of the shelves hold
children’s books. which type of book covers the most shelf space in the
store? explain how you arrived at your answer.

Mysteries ; 0.3 is 30/100 =30%
travel ; 25%
children ; 7/20 (5/5) = 35/100 = 35%
children's books cover the most shelf

There are multiple ways to answer this, because one would have to convert all of the numbers into a percentage, decimal, or fraction. Let's go with percentage.
      Travel books = 25% is already a percentage, so you don't have to worry about it.
      Mystery books = 0.3 is easy to convert. 1 = 100%, so 0.3 = 30%. See? Easy.
      Children books = 7/20 is the hardest, although it isn't THAT hard. Any percentage means "this number out of a hundred", and 20 is one-fifth of one hundred. So, we can multiply the fraction by one, essentially, in this form: 7/20 * 5/5. This equals 35/100, which is 35%.
We have 25%, 30%, and 35%. Children's books take up 35%, which is the largest amount.

Comment if you want me to convert them into either decimals or fractions (or both).

21 is the answer the answer is 21