Question 802465
Let {{{ w }}} = gallons of water to be added
{{{ .5*500 = 250 }}}  gallons of sodium hydroxide
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{{{ 250 / ( 500 + w ) = .4 }}}
{{{ 250 = .4*( 500 + w ) }}}
{{{ 250 = 200 + .4w }}}
{{{ .4w = 50 }}}
{{{ w = 125 }}}
125 gallons of water must be added