a, ( 20 + \(9\dfrac{1}{4}\) ) : \(2\dfrac{1}{4}\)
= ( 20 + \(\dfrac{37}{4}\) ) : \(\dfrac{9}{4}\)
= ( \(\dfrac{80}{4}\) + \(\dfrac{37}{4}\) ) . \(\dfrac{4}{9}\)
= \(\dfrac{117}{4}\) . \(\dfrac{4}{9}\)
= \(\dfrac{117}{9}\) = 13
b, ( 6 - \(2\dfrac{4}{5}\) ) . \(3\dfrac{1}{8}\) - \(1\dfrac{3}{5}\) : \(\dfrac{1}{4}\)
= ( 6 - \(\dfrac{14}{5}\) ) . \(\dfrac{25}{8}\) - \(\dfrac{8}{5}\) . 4
= ( \(\dfrac{30}{5}\) - \(\dfrac{14}{5}\) ) . \(\dfrac{25}{8}\) - \(\dfrac{8}{5}\) . 4
= \(\dfrac{16}{5}\) . \(\dfrac{25}{8}\) - \(\dfrac{8}{5}\). 4
= 10 - \(\dfrac{32}{5}\)
= \(\dfrac{50}{5}\) - \(\dfrac{32}{5}\)
= \(\dfrac{18}{5}\)
c, \(\dfrac{32}{15}\) : ( -\(1\dfrac{1}{5}\) + \(1\dfrac{1}{3}\) )
= \(\dfrac{32}{15}\) : ( \(\dfrac{-6}{5}\) + \(\dfrac{4}{3}\) )
= \(\dfrac{32}{15}\) : ( \(\dfrac{-18}{15}\) + \(\dfrac{20}{15}\) )
= \(\dfrac{32}{15}\) : \(\dfrac{2}{15}\)
= \(\dfrac{32}{15}\) . \(\dfrac{15}{2}\)
= 16
