the cheap answer is just to "multiply each fraction by the other's denominator". So let's do so,
[tex]\bf \cfrac{3}{4}\cdot \cfrac{10}{10}\implies \cfrac{30}{\boxed{40}}\qquad \qquad \qquad \qquad \qquad \qquad \cfrac{9}{10}\cdot \cfrac{4}{4}\implies \cfrac{36}{\boxed{40}}[/tex]
