a) $\dfrac{-8}{18}$ - $\dfrac{15}{27}$ + $\dfrac{1}{2}$
= $\dfrac{-4}{9}$ - $\dfrac{5}{9}$ + $\dfrac{1}{2}$
= $-1$ + $\dfrac{1}{2}$
= $\dfrac{-1}{2}$
c) $\dfrac{27}{23}$ + $\dfrac{5}{21}$ - $\dfrac{4}{23}$ + $\dfrac{16}{21}$ - $\dfrac{1}{2}$
= ($\dfrac{27}{23}$ + $\dfrac{-4}{23}$) + ($\dfrac{5}{21}$ + $\dfrac{16}{21}$) + $\dfrac{-1}{2}$
= $1$ + $1$ + $\dfrac{-1}{2}$
= $2$ + $\dfrac{-1}{2}$
= $\dfrac{3}{2}$
