Question 809720
It really helps if she assigns a variable to the
thing they want her to find and write it down.
In this case:
Let {{{ a }}} = pounds of Grade A coffee needed
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Now she can also write down an expression for
the cost of the coffee blend. She should write it
down also. It is:
{{{ 9a + 6*20 }}}
This is just ( cost/pound )x( pounds ) + ( cost/pound )x( pounds )
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Now she can go to the final step which is:
( total cost of the blend ) / ( total pounds of the blend ) = ( given cost/pound )
{{{ ( 9a + 6*20 ) / ( a + 20 ) = 7 }}}
Multiply both sides by {{{ a + 20 }}}
{{{ 9a + 120 = 7*( a + 20 ) }}}
{{{ 9a + 120 = 7a + 140 }}}
{{{ 2a = 20 }}}
{{{ a = 10 }}}
10 pounds of grade A coffee are needed
check answer:
{{{ ( 9a + 6*20 ) / ( a + 20 ) = 7 }}}
{{{ ( 9*10 + 6*20 ) / ( 10 + 20 ) = 7 }}} 
{{{ ( 90 + 120 ) / 30 = 7 }}}
{{{ 90 + 120 = 210 }}}
{{{ 210 = 210 }}}
OK
Hope this helps