$\text{a) $\dfrac{3}{4}+\dfrac{2}{5}+\dfrac{1}{4}+\dfrac{3}{5}$.}$
= `\frac{3}{4}`+`\frac{1}{4}`+`\frac{2}{5}`+`\frac{3}{5}`.
= `\frac{4}{4}`+`\frac{5}{5}`.
$\text{= 1 + 1.}$
$\text{= 2.}$
$\text{b) $\dfrac{2}{3}×\dfrac{3}{4}×\dfrac{4}{5}×\dfrac{5}{6}$}$
$\text{= $\dfrac{2×3×4×5}{3×4×5×6}$}$
$\text{= $\dfrac{2}{6}$}$
$\text{= $\dfrac{1}{3}$}$
$\text{c) $\dfrac{4}{5}-\dfrac{2}{5}+\dfrac{1}{5}-\dfrac{1}{3}$.}$
$\text{= $\dfrac{4-2+1}{5}-\dfrac{1}{3}$.}$
$\text{= $\dfrac{3}{5}-\dfrac{1}{3}$.}$
$\text{= $\dfrac{9}{15}-\dfrac{5}{15}$.}$
$\text{= $\dfrac{4}{15}$.}$
$\text{d) $\dfrac{2006}{2005}×\dfrac{3}{4}-\dfrac{3}{4}×\dfrac{1}{2005}$}$
$\text{= $\dfrac{3}{4}$×($\dfrac{2006}{2005}-\dfrac{1}{2005})$}$
$\text{= $\dfrac{3}{4}$×$\dfrac{2005}{2005}$}$
$\text{= $\dfrac{3}{4}$×1}$
$\text{= $\dfrac{3}{4}$}$
