[tex]\displaystyle \text{Factor comun pe 13.} \\ \\ 13+26+39+...+715= \\ 13(1+2+3+...+55)= \\ \\ \text{Aplicam Suma lui Gass in paranteza} \\ \\ 1+2+3+...+55= \frac{55(55+1)}{2}= \frac{55\cdot\not56}{\not2}= 55\cdot 28=1540 \\ \\ 13 \cdot 1540=\boxed{20.020}[/tex]
