(5+5+5) + (1+1+1) = 18
[tex]\it Not\breve a m:\\ \\ a\ -\ num\breve a rul\ bancnotelor\ de\ 1\ leu;\\ \\ b\ -\ num\breve a rul\ bancnotelor\ de\ 5\ lei.\\ \\ a=b\ \ \ \ \ (*) \\ \\ a\cdot1+b\cdot5=18 \Rightarrow a+5b=18\ \stackrel{(*)}{\Longrightarrow}\ a+5a=18 \Rightarrow\\ \\\Rightarrow 6a=18\Big|_{:6} \Rightarrow a=3\\ \\ Deci,\ \ a=b=3[/tex]
