Question 153484
Colombian $3.00/lb, let's use C as the number of pounds of Colombian.
Costa Rica $6.00/lb, let's use R as the number of pounds of Costa Rican.
We want a total 90 pounds so we know,
1.{{{C+R=90}}}
We also want to sell it for $4.00/lb. 
The average price will be cost of the Colombian plus the cost of the Costa Rican divided by the total number of pounds. 
2.{{{(3C+6R)/(C+R)=4}}}
From 1, we can substitute into 2,
2.{{{(3C+6R)/(C+R)=4}}}
2.{{{(3C+6R)/90=4}}}
2.{{{3C+6R=360}}}
Use eq. 1 to solve for C in terms of R.
1.{{{C+R=90}}}
{{{C=90-R}}}
Now substittue that expression into eq. 2 and solve for R.
2.{{{3C+6R=360}}}
{{{3(90-R)+6R=360}}}
{{{270-3R+6R=360}}}
{{{3R=90}}}
{{{highlight(R=30)}}}
From above,
{{{C=90-R}}}
{{{C=90-30}}}
{{{highlight(C=60)}}}
Let's check the per pound price to verify the answer.
{{{3C+6R=360}}}
{{{3(60)+6(30)=360}}}
{{{180+180=360}}}
{{{360=360}}}
True, good answer.
60 pounds Colombian, 30 pounds Costa Rican.