Răspuns :
[tex] \frac{a}{2} = \frac{b}{5} \\ b= \frac{5a}{2} \\
\frac{x}{100}*a=b \\ \frac{x}{100}*a= \frac{5a}{2}
\\ x= \frac{5a}{2}* \frac{1}{a}*100= \frac{500}{2} =250[/tex]
[tex]\frac{a}{2}=\frac{b}{5} \,conf. proprietatiilor\, proportiilor\, derivate\,,\\
cu \,aceleasi \,termeni\;\rightarrow\;\frac{a}{b}=\frac{^{20)}2}{5}=\frac{40}{100}=40^o/_o[/tex]