Final answer:
Dr. Drange has 55 romantic novels.
Step-by-step explanation:
To find the number of romantic novels that Dr. Drange has, we need to subtract the number of math and science books from the total. First, we find the number of math books by multiplying 90 books by 2/9. This gives us 20 math books. Then, we find the number of science books by multiplying 90 books by 1/6. This gives us 15 science books. Finally, we subtract the sum of math and science books from the total to get the number of romantic novels. So, Dr. Drange has 90 - 20 - 15 = 55 romantic novels.