Question 585850
Let {{{ x }}} = liters of pure alcohol to be added
{{{ .7*10 = 7 }}} liters of alcohol in 70% solution
In words:
( liters of alcohol you end up with ) / ( total liters of solution ) = 90%
{{{ ( x + 7 ) / ( 10 + x ) = .9 }}}
{{{ x + 7 = .9*( 10 + x ) }}}
{{{ x + 7 = 9 + .9x }}}
{{{ .1x = 2 }}}
{{{ x = 2/.1 }}}
{{{ x = 20 }}}
20  liters of pure alcohol must be added