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EX:37,38

37.
a+b+c+d=138
a=b+c
b=c:2
d=30

a+b+c+30=138 =>a+b+c=108
Inlocuim pe a in aceasta expresie si avem b+c+b+c=108 sau b+c=54
Stim ca c=2b =>b+2b=54 sau 3b=54 =>b=18 =>c=36
Deci a=18+36 =>a=54

38.
a+b+c=45
a=3c
b=c:2 
3c+c:2+c=45 Inmultim totul cu 2 si avem 6c+c+2c=90 =>9c=90 =>c=10
a=30 si b=5

a+b+c+d=138
a+b+c=30=138
a+b+c=138-30
a+b+c=108
a=b+c
b=2c inlocuim si avem (b+c)+2c+c=108 b+4c+=108 b=108-4c
si stim ca b=2c deci  108-4c= 2c si 108=6c c = 108:6=18 c=18
b=2x18=36
a=18+36=54 
verific 54+36+18+30=138