Question 798032
Focus on the fractions.  You are dividing whole 1 quantity of the "rs".  

x for youngest
y for middle aged
z for eldest


{{{z=2x}}} and {{{z=(3/2)y}}};


{{{x+y+z=1}}}



Three equations in three unknown variables.  Solve any way you want.


A method can be:  Use the first equation for z and substitute into the other two equations.  
{{{2x=(3/2)y}}} and {{{x+y+2x=1}}};
{{{y=(2/3)2x}}} and {{{3x+y=1}}};
{{{y=(4/3)x}}} and  {{{3x+y=1}}}.
Take this formula for y and substitute into the other equation;
{{{3x+(4/3)x=1}}}
solve for x,
{{{9x/3+4x/3=1}}}
{{{13x/3=1}}}
{{{highlight(x=3/13)}}}


Going back to the original first two equations, you would use x to find z, and use z to find y.