Explicație pas cu pas:
a+b=400
b+c=600
b+a=200
----------------
2a+2b+2c=1200
2(a+b+c)=1200
a+b+c=1200:2
a+b+c=600
----------—-------
400+c=600
c=600 - 400
c=200
a+600=600
a=0
200+c=600
c=600-200
c=400
