[tex]4x = 5y \: | \div y[/tex]
[tex] \frac{4x}{y} = 5[/tex]
[tex]4 \times \frac{x}{y} = 5[/tex]
[tex] \frac{x}{y} = \frac{5}{4} [/tex]
[tex] \frac{3x}{10y} = \frac{3}{10} \times \frac{x}{y} = \frac{3}{10} \times \frac{5}{4} = \frac{3 \times 5}{10 \times 4} = \frac{15}{40} = \frac{3}{8} [/tex]
