Question 752579
Let {{{ x }}} = gallons of pure acid to be added
{{{ .4*5 = 2 }}} gallons of acid in 40% solution
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{{{ ( 2 + x ) / ( 5 + x ) = .9 }}}
{{{ 2 + x = .9*( 5 + x ) }}}
{{{ 2 + x = 4.5 + .9x }}}
{{{ .1x = 2.5 }}}
{{{ x = 25 }}}
25 gallons of pure acid must be added
check:
{{{ ( 2 + x ) / ( 5 + x ) = .9 }}}
{{{ ( 2 + 25 ) / ( 5 + 25 ) = .9 }}}
{{{ 27 / 30 = .9 }}}
{{{ 27 = .9*30 }}}
{{{ 27 = 27 }}}
OK