[tex]a = \dfrac{40}{100}\cdot b \\ \\ b= \dfrac{20}{100} \cdot c \\ \\ ($inlocuim pe b in a)$ \Rightarrow a = \dfrac{40}{100}\cdot \Big(\dfrac{20}{100} \cdot c\Big) \Rightarrow a = \dfrac{4\cdot 2}{100}\cdot c \Rightarrow \\ \\ \Rightarrow a = \dfrac{4}{50}\cdot c \Rightarrow a = \dfrac{2}{25}\cdot c \Rightarrow \dfrac{2}{25} = \dfrac{a}{c} \Rightarrow \boxed{\dfrac{a}{c}= \dfrac{2}{25} } [/tex]