A student has sat an exam in $$8$$ subjects and has obtained $$5$$ passes and $$3$$ failures. Calculate the percentage of every note.
Development:
It is a question of applying rules of three to know the percentage of every result.
In the case of the passes: $$\begin{eqnarray} 5 & \rightarrow & 8\\\\ x & \rightarrow & 100 \end{eqnarray}$$
So,he has passed $$5$$ of $$8$$, which represents a $$x$$ per cent.
Expressed in fractions it is:
$$\dfrac{5}{x}=\dfrac{8}{100} \Rightarrow 8x=5\cdot100 \Rightarrow 8x=500 \Rightarrow x=\dfrac{500}{8}=62,5\%$$
In case of the failures: $$\begin{eqnarray} 3 & \rightarrow & 8\\\\ x & \rightarrow & 100 \end{eqnarray}$$
Otherwise:
$$\dfrac{3}{x}=\dfrac{8}{100} \Rightarrow 8x=3\cdot100 \Rightarrow 8x=300 \Rightarrow x=\dfrac{300}{8}=37,5\%$$
It was possible to find the same value by substracting the entire percentage of passes from $$100\%$$:
$$100-62,5=37,5\%$$
Solution:
The percentage of passes is $$62,5\%$$ and the one of failures is $$37,5\%$$.