If you have a decimal you would move it to the right two numbers like .56 to 56. percent or you have a fraction you would put a decimal like 4/5 to 4/5.= 80.percent.
The equation of a direct variation is of the form

But the given function can be rewritten as

This is a partial variation,

is the constant of variation and

is the fixed constant.
Answer:
No, the books had more sales by 5/6 being bigger (Greater Than) than 3/4.
The different between n and 15 is 5 in an algebraic expression would be n-16=5
the solution would be n=21