## Basic College Mathematics (9th Edition)

$\frac{3}{4}$
The LCD of the original denominators, 10 and 20, is 20. Therefore, we can rewrite both fractions with a denominator of 20. $\frac{9}{20}+\frac{3}{10}=\frac{9}{20}+\frac{3\times2}{10\times2}=\frac{9}{20}+\frac{6}{20}=\frac{9+6}{20}=\frac{15}{20}=\frac{3}{4}$