Question 723060
x = amount sand
y = amount cement
z = amount gravel

x/y=2. This means x=2y.  Also, means y/x=1/2, so {{{y=x/2}}}.
z/x=1.5.  This means {{{z=1.5x}}}.


Want 9 yd^3 of concrete.
{{{x+y+z=9}}}.
Using the above substitutions allows to express everything in terms of just x.
{{{x+(x/2)+((3/2)x)=9}}}
{{{(1+1/2+3/2)x=9}}}
...continue from here to solve and finish for x, y, and z.