 \frac{42}{15} * \frac{9}{12} =  \frac{3*14}{3*5} * \frac{3*3}{3*4} = \frac{14}{5} * \frac{3}{4} = \frac{14*3}{5*4} = \frac{42}{20} =  \frac{2*21}{2*10} =  \frac{21}{10} 