Is this a valid conclusion?

On a piece of paper graph f(x)=5•(0.4)^x

Select the correct answer. joshua likes to read. he read 6 books when he was 6 years old. every year he doubled the number of books he read the previous year. how many total books did he read between the ages of 6 and 10? a. 60 b. 90 c.186 d.187

How can you tell when x and y are not directly proportional?