Dr. Drange has a collection of 90 books. Of the books, 2/9 are math books, 1/6 are science books, and the rest are romantic novelss. How many romantic novels does he have?

13/18 used common denominator of 90 so 2/9 x 10 and 1/6 x 15 gave me 20/90 and 15/90. Added those and cane up with 35/90. Difference was 65/90 so but could be divided by 5 so my answer came out to 13/18.