Aubrey has a shelf full of books exactly one third of books on the shelf are mysteries. aubrey has read 10 of the mysteries on the shelf. the number of mysteries aubrey has read is greater than 1/5 of the number of mysteries on the shelf and less than 1/4 of the number of mysteries on the shelf. what is the number of books on the shelf?

The correct answer for this question is this one: "147"

Here's how to solve this problem.

Let  x = number of mystery books on the shelf 
Let 3x = total number of books on the shelf 

we are told that 
x/5<10<x/4 
multiply by 20, 
4x<200<5x 
which means that 
40<x<50 
So the number of mystery books can be any number between 41 and 49.
In other words, the possible total number of books could be three times the above numbers, 123, 126, 129, 
Choose the one that suits your answer choices.

Here are the following choices:
Which could be the number of books on the shelf? 
A. 120 B 140 c 147 D 150