[tex]\displaystyle a). \frac{15}{y} = \frac{5}{8} \Rightarrow 15 \cdot 8=5y \Rightarrow 120=5y \Rightarrow y= \frac{120}{5} \Rightarrow y=24 \\ \\ b). \frac{18}{7} = \frac{z}{9} \Rightarrow 18 \cdot 9=7z \Rightarrow 162=7z \Rightarrow z= \frac{162}{7} \\ \\ c). \frac{4}{13} = \frac{3}{t} \Rightarrow 4t=13 \cdot 3 \Rightarrow 4t=39 \Rightarrow t= \frac{39}{4} \\ \\ d). \frac{u}{20} = \frac{7}{9} \Rightarrow 9u=20 \cdot 7 \Rightarrow 9u=140 \Rightarrow u= \frac{140}{9} [/tex]
