Chapter 2 - Real Numbers - 2.4 - Exponents - Problem Set 2.4 - Page 75: 59

$-\dfrac{11n^2}{60}$

Because the least common denominator is $60$, we create a common denominator and obtain: \begin{array}{l}\require{cancel} \dfrac{20(2n^2)-15(1n^2)-12(3n^2)}{60} \\\\= \dfrac{40n^2-15n^2-36n^2}{60} \\\\= \dfrac{-11n^2}{60} \\\\= -\dfrac{11n^2}{60} .\end{array}