\[A = \dfrac{15}{21}\times \dfrac{14}{12} = \dfrac{5}{7}\times\dfrac{7}{6} =\dfrac{5\times 7}{7\times 6} = \dfrac{5}{6}.\] \[B = \dfrac{3-\dfrac 2 5}{2} =\dfrac{\dfrac{15}5 - \dfrac 2 5}{2} =\dfrac{\dfrac{13}5}{2} =\dfrac{13} 5 \times \dfrac 1 2 = \dfrac{13}{10}.\] \[C = \dfrac 1{\dfrac 3 4 + \dfrac 1 2} = \dfrac 1 {\dfrac 3 4 + \dfrac 2 4} =\dfrac 1 {\dfrac 5 4} =\dfrac 4 5.\]