Najpierw obliczmy wartości liczbowe a i b.

$a=\left(1\frac{1}{3}-0,5\right):\left(\frac{7}{12}-\frac{1}{4}\right)=\left(\frac{4}{3}-\frac{1}{2}\right):\left(\frac{7}{12}-\frac{3}{12}\right)=\left(\frac{8}{6}-\frac{3}{6}\right):\frac{4}{12}=\frac{5}{6}:\frac{1}{3}=\frac{5}{6}\cdot \frac{3}{1}=\frac{15}{6}=\frac{5}{2}=2,5$