[tex] \frac{3}28} [/tex]+[tex] \frac{1}{72} [/tex]+[tex] \frac{5}{84} [/tex]=
=[tex] \frac{9}{84} [/tex]+[tex] \frac{1}{72} [/tex]+[tex] \frac{5}{84} [/tex]
=[tex] \frac{14}{84} [/tex]+[tex] \frac{1}{72} [/tex]
=[tex] \frac{1}{6} [/tex]+[tex] \frac{1}{72} [/tex]
=[tex] \frac{12}{72} [/tex]+[tex] \frac{1}{72}[/tex]
=[tex] \frac{13}{72} [/tex]