(a+b+c):2=600
a+b+c=600*2
a+b+c=1200
(a+b):2=220
a+b=440
a+b+c=1200
440+c=1200
c=1200-440
c=760
a=c:5 (primul nr este 1/5 din al 3-lea)
a=760:5
a=152
a+b+c=1200
152+b+760=1200
b+760+152=1200
b+912=1200
b=1200-912
b=288
Deci, a=152, b=288, c=760
Proba: a+b+c=1200
152+288+760=1200
440+760=1200 : Adevarat
