Răspuns:
5 +10 + 15+...+2005+2010+2015
2015+2010+2005+...+15 + 10 + 5
------------------‐--------------------------------‐----------+
2020+2020+2020+...2020+2020+2020
5*1=5
5*2=10
5*3=15
'
'
'
5*401=2005
5*402=2010
5*403=2015
2S=2020x401
2s=810020
s=810020:2
s=405010
Explicație pas cu pas:
P.S NUSH DACA E BN