Question 751561
{{{ .18*20 = 3.6 }}} = liters of punch in 18% punch mix
{{{ .3*50 = 15 }}} = liters of punch in 30% punch mix
Let {{{ w }}} = liters of water to be added to get 25% punch mix 
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{{{ ( 3.6 + 15 ) / ( 20 + 50 + w ) = .25 }}}
{{{ 18.6 / ( 70 + w ) = .25 }}}
{{{ 18.6 = .25*( 70 + w ) }}}
{{{ 18.6 = 17.5 + .25w }}}
{{{ .25w = 1.1 }}}
{{{ w = 4.4 }}}
4.4 liters of water should be added
check:
{{{ 18.6 / ( 70 + w ) = .25 }}}
{{{ 18.6 / ( 70 + 4.4 ) = .25 }}}
{{{ 18.6 = .25*74.4 }}}
{{{ 18.6 = 18.6 }}}
OK