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a+b=300 ;b+c=500 ;a+c=400

Răspuns :

a+b=300
b+c=500
a+c=400
2a+2b+2c=1200
2(a+b+c)=1200
a+b+c=1200:2=600

a+b+c=600
a+b=300
c=600-300
c=300
b+c=500
b+300=500
b=500-300
b=200
a+c=400
a+300=400
a=400-300
a=100