\(a,=\dfrac{20}{60}+\dfrac{12}{60}-\dfrac{15}{60}\\ =\dfrac{17}{60}\\ b,=\dfrac{7}{8}-\left(\dfrac{5}{20}+\dfrac{8}{20}\right)\\ =\dfrac{7}{8}-\dfrac{13}{20}\\ =\dfrac{35}{40}-\dfrac{26}{40}=\dfrac{9}{40}\\ c,=\dfrac{5}{11}-\dfrac{3}{55}=\dfrac{25}{55}-\dfrac{3}{55}=\dfrac{22}{55}=\dfrac{2}{5}\)
a: =20/60+12/60-15/60=17/60
b: =7/8-1/4-2/5=5/8-2/5=25/40-16/40=9/40
c: =5/11-3/5+6/11=1-3/5=2/5