a) [tex]\frac{3}{\not7\,1}\cdot\not28\,4=\frac{3\cdot4}{1}=\boxed{12}[/tex]
b) [tex]\frac{1}{3}\cdot3\frac{3}{4}=\frac{1}{\not3\,1}\cdot\frac{\not15\,5}{4}=\frac{1\cdot5}{1\cdot4}=\boxed{\frac{5}{4}=1\frac{1}{4}}[/tex]
c) [tex]1\frac{1}{7}\cdot2\frac{1}{4}=\frac{\not8\,2}{7}\cdot\frac{9}{\not4\,1}=\frac{2\cdot9}{7\cdot1}=\boxed{\frac{18}{7}=2\frac{4}{7}}[/tex]
