18) (2x+6y)/(25x-y)=58/41
41(2x+6y)=58(25x-y)
82x+246y=1450x-58y
1450x-82x=246y+58y
1368x=304y /:152
9x=2y rezulta ca x/2=y/9
21) a/3=b/5=c/7
Ma=(a+b+c)/3=10
a/3=b/5=c/7=k
a=3k
b=5k
c=7k
Ma=(3k+5k+7k)/3=10
15k=30
k=2 a=3*2=6
b=5*2=10
c=7*2=14
proba Ma=(6+10+14)/3=30/3=10