Question 993897
{{{ .72*3 = 2.16 }}} = liters of acid in 72% solution
Let {{{ x }}} = liters of 9% solution needed
{{{ .09x }}} = liters of acid in 9% solution
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{{{ ( 2.16 + .09x ) / ( 3 + x ) = .36 }}}
{{{ 2.16 + .09x = .36*( 3 + x ) }}}
{{{ 2.16 + .09x = 1.08 + .36x }}}
{{{ .27x = 1.08 }}}
{{{ x = 4 }}}
4  liters of 9% solution are needed
check:
{{{ ( 2.16 + .09x ) / ( 3 + x ) = .36 }}}
{{{ ( 2.16 + .09*4 ) / ( 3 + 4 ) = .36 }}}
{{{  2.16 + .36 = .36*7 }}}
{{{ 2.52 = 2.52 }}}
OK