Let \(x\) be the number of multiple choice questions and \(y\) the number of short-answer questions.
Now,
\(\begin{cases}x+y=25\\2x+4y=80\end{cases}\)
Since you only care about finding \(y\), you can substitute the first equation into the second to eliminate \(x\). You have
\(x+y=25\implies x=25-y\)
and substituting into the other equation,
\(2(25-y)+4y=80\)
\(50-2y+4y=80\)
\(2y=30\)
\(y=15\)
