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------------------------------------------------ Which of the following proportions is false?

A. $\frac{12}{15} = \frac{20}{25}$

B. $\frac{25}{45} = \frac{50}{90}$

C. $\frac{20}{50} = \frac{40}{100}$

D. $\frac{18}{48} = \frac{30}{50}$

To determine which of the given proportions is false, rewrite the fractions on both sides each equation so that the denominators are the same.

[tex]\dfrac{12}{15}=\dfrac{20}{25} \implies \dfrac{12 \div 3}{15 \div 3}=\dfrac{20 \div 5}{25 \div 5}\implies \dfrac{4}{5}=\dfrac{4}{5}\qquad \boxed{\sf True}[/tex]

[tex]\dfrac{25}{45}=\dfrac{50}{90} \implies \dfrac{25 \div 5}{45\div 5}=\dfrac{50\div 10}{90\div 10}\implies \dfrac{5}{9}=\dfrac{5}{9}\qquad \boxed{\sf True}[/tex]

[tex]\dfrac{20}{50}=\dfrac{40}{100} \implies \dfrac{20\div 10}{50\div 10}=\dfrac{40\div20}{100\div 20}\implies \dfrac{2}{5}=\dfrac{2}{5}\qquad \boxed{\sf True}[/tex]

[tex]\dfrac{18}{48}=\dfrac{30}{50} \implies \dfrac{18\times 25}{48\times 25}=\dfrac{30\times 24}{50 \times24}\implies \dfrac{450}{1200}= \dfrac{720}{1200}\qquad \boxed{\sf False}[/tex]

Therefore, the proportion that is false is 18/48 = 30/50.