Question 1030336
Almost a simple proportion.


c for amount of cement mix
g for amount of gravel
{{{c+g=90}}}


The proportion you might want to arrange is using {{{(5/1)(poundsCementMix/poundsGravel)}}}.  Using this ratio to set a proportion will mean you may use g as your main variable, and then {{{c=90-g}}}. 


{{{highlight_green((5/1)(cementmix/gravel)=((90-g)/g)(cementmix/gravel))}}}


Upon seeing the units taken care well,
{{{5=(90-g)/g}}};
Go from there.....