If original quantity is 15 and a new quantity is 24 what is the percent increase

Assuming the 15 and 24 is dollars, 
To find Markup % use the following equation
Markup%=Markup/Cost*100
and 
Markup=Selling Price-Cost

Markup=24-15= $9

The Markup is now $9

Now, Markup%= 9/15
=0.6*100
=60

Thus, the markup was 60%.