a,
x + $\dfrac{1}{2}$ × x = $\dfrac{4}{5}$
x × 1 + $\dfrac{1}{2}$ × x = $\dfrac{4}{5}$
x × ( $\dfrac{1}{2}$ + 1 ) = $\dfrac{4}{5}$
x × $\dfrac{3}{2}$ = $\dfrac{4}{5}$
x = $\dfrac{4}{5}$ ÷ $\dfrac{3}{2}$
x = $\dfrac{8}{15}$
b,
x × 2 + x × $\dfrac{1}{5}$ = 1$\dfrac{1}{3}$
x × ( 2 + $\dfrac{1}{5}$ ) = $\dfrac{4}{3}$
x × $\dfrac{11}{5}$ = $\dfrac{4}{3}$
x = $\dfrac{4}{3}$ ÷ $\dfrac{11}{5}$
x = $\dfrac{20}{33}$
