\(\dfrac{3}{15}=\dfrac{a}{20}=\dfrac{24}{b}\)\(\Rightarrow\left\{{}\begin{matrix}3b=24.15\\15a=3.20\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}b=120\\a=4\end{matrix}\right.\)

\(\Rightarrow a+b=4+120=124\)

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