23. a) [tex] \frac{5}{21} - \frac{3}{14}+ \frac{2}{7}= \frac{5*2}{21*2}- \frac{3*3}{14*3}+ \frac{2*6}{7*6}=\frac{10}{42} - \frac{9}{42}+ \frac{12}{42}= \frac{10-9+12}{42}= \frac{13}{42} [/tex]
b) [tex]\frac{9}{20} + \frac{7}{10}- \frac{2}{15}=\frac{9*3}{20*3}+ \frac{7*6}{10*6}- \frac{2*4}{15*4}=\frac{27}{60} + \frac{42}{60}- \frac{8}{60}= \frac{27+42-8}{60}= \frac{61}{60} [/tex]
c) [tex]\frac{11}{20} -( \frac{1}{4}+ \frac{1}{5})=\frac{11}{20}- \frac{9}{20}=\frac{11-9}{20}= \frac{2}{20}= \frac{1}{10} [/tex]
