http://www.mathe24.net/fakultaet.html?action=do
$${\frac{\left({\frac{{\mathtt{3}}}{{\mathtt{4}}}}\right)}{\left({\frac{{\mathtt{5}}}{{\mathtt{4}}}}\right)}} = \left({\frac{{\mathtt{3}}}{{\mathtt{4}}}}\right){\mathtt{\,\times\,}}\left({\frac{{\mathtt{4}}}{{\mathtt{5}}}}\right) = {\frac{{\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{4}}}{\left({\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{5}}\right)}} = {\frac{{\mathtt{3}}}{{\mathtt{5}}}}$$
http://calculator.intemodino.com/de/bruchrechner.html
http://www.arndt-bruenner.de/mathe/scripts/geradengleichungen1.htm
http://www.geogebratube.org/material/show/id/362835
http://www.geogebratube.org/student/m54117