Question 58286
Let a = amount of the $9/pound beans in mixture (in pounds)
Let b = amount of the $12/pound beans in mixture (in pounds)
a + b = 100 (this is given)
Set up this equation:
{{{(9*a + 12*b) / 100 = 11.25 / 1}}}
This simply says "The cost of a pounds of the $9 beans plus 
the cost of b pounds of the $12 beans per 100 pound bag must
equal $11.25 per 1 pound."
{{{9a + 12b = 1125}}}
{{{ a + b = 100}}} (given)
{{{12a + 12b = 1200}}} (multiply each side by 12)
Now subtract the 1st equation from the second
{{{3a = 75}}}
{{{a = 25}}} (amt of the %9 beans to use)
{{{b = 100 - 25}}}
{{{b = 75}}} (amt of the $12 beans to use)
check
{{{(9*a + 12*b) / 100 = 11.25 / 1}}}
{{{(9*25 + 12*75) / 100 = 11.25 / 1}}}
do the math- checks OK