<PRE><B>Question 417064</B>
Let {{{a}}} = ounces of 6% solution needed
Let {{{b}}} = ounces of 12% solution needed
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given:
(1) {{{a + b = 24}}}
(2) {{{ (.06a + .12b)/24 = .07 }}}
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{{{.06a + .12b = .07*24}}}
{{{.06a + .12b = 1.68 }}}
(2) {{{ 6a + 12b = 168 }}}
Multiply both sides of (1) by  {{{6}}}
and subtract from (2)
(2) {{{ 6a + 12b = 168 }}}
(1) {{{-6a - 6b = -144}}}
{{{6b = 24}}}
{{{b = 4}}}
and
{{{a = 24 - 4}}}
{{{a = 20}}}
She needs 20 ounces of 6% solution and
4 ounces of 12% solution</PRE>