\(\dfrac{30-x}{30}=\dfrac{8}{15}\)
\(\Rightarrow\left(30-x\right)15=30\times8\)
\(\Rightarrow\left(30-x\right)15=240\)
\(\Rightarrow30-x=240:15\)
\(\Rightarrow30-x=16\)
\(\Rightarrow x=30-16\)
\(\Rightarrow x=14\)
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\(\dfrac{x+30}{72}=\dfrac{5}{8}\)
\(\Rightarrow\left(x+30\right)8=5\times72\)
\(\Rightarrow\left(x+30\right)8=360\)
\(\Rightarrow x+30=360:8\)
\(\Rightarrow x+30=45\)
\(\Rightarrow x=45-30\)
\(\Rightarrow x=15\)