Chapter 1 - Section 1.7 - Addition and Subtraction of Fractions - Exercise - Page 40: 21

$\frac{13}{120}$

Find the LCD. $\ \ \ \ \ 20=10\times2$ $\ \ \ \ \ 30=10\times\ \ \ \ \ \ \ \ \ \ \ 3$ $\ \ \ \ \ 40=10\times2\times2$ $LCD=10\times2\times2\times3=120$ $\frac{1}{20}+\frac{1}{30}+\frac{1}{40}=\frac{1\times6}{20\times6}+\frac{1\times4}{30\times4}+\frac{1\times3}{40\times3}=\frac{6}{12}+\frac{4}{120}+\frac{3}{120}=\frac{6+4+3}{120}=\frac{13}{120}$