-Determinati numerele rationale pozitive a, b, c, stiind ca:
a)a/2=b/3=c/5 si a+b+c=15
b) a/3=b/5=c/9 si 3a +b-c=20
c) a/b=4/9=c/3, b ≠0 si 4a-b+c=20