the cheap answer is, you simply "grab the denominator of one and multiply it times the other's top and bottom", so let's do so,
[tex]\bf \cfrac{3}{5}\cdot \cfrac{4}{4}\implies \cfrac{12}{20}\qquad \qquad \qquad \qquad \qquad \cfrac{3}{4}\cdot \cfrac{5}{5}\implies \cfrac{15}{20}[/tex]
