\(\Rightarrow5x-4x=100\Leftrightarrow x=100\)
\(\dfrac{x}{40}-\dfrac{x}{50}=\dfrac{1}{2}\\ \Rightarrow x\left(\dfrac{1}{40}-\dfrac{1}{50}\right)=\dfrac{1}{2}\\ \Rightarrow x.\dfrac{1}{200}=\dfrac{1}{2}\\ \Rightarrow x=\dfrac{1}{2}:\dfrac{1}{200}\\ \Rightarrow x=100\)