<PRE><B>Question 186415</B>
Let {{{a}}}= pounds of $20 coffee used
given:
{{{20a}}}= cost in dollars of $20 coffee used
{{{80*90 = 7200}}} = cost in dollars of $90 coffee used
{{{a + 80}}} = pounds of coffee in final mixture
{{{60*(a + 80)}}} = cost in dollars of final mixture
-------------------
{{{20a + 7200 = 60*(a + 80)}}}
{{{20a + 7200 = 60a + 4800}}}
{{{40a = 2400}}}
{{{a = 60}}}
60 pounds of $20 coffee must be used
check:
{{{20a + 7200 = 60*(a + 80)}}}
{{{20*60 + 7200 = 60*(60 + 80)}}} 
{{{1200 + 7200 = 60*140}}}
{{{8400 = 8400}}}
OK </PRE>