[tex]\it 11+22+33+\ ...\ + 99= 11(1+2+3+\ ...\ + 9) = 11\cdot\dfrac{9\cdot10}{2}=
\\ \\ \\
= 11\cdot9\cdot5= 99\cdot5[/tex]
Proporția devine:
[tex]\it \dfrac{99\cdot5}{99} = \dfrac{x}{4} \Rightarrow 5 = \dfrac{x}{4} \Rightarrow \dfrac{x}{4} = 5 \Rightarrow x=4\cdot5\Rightarrow x = 20.[/tex]
