Question 714223
Let {{{ w }}} = liters of distilled water to be used
Let {{{ x }}} = liters of 70% alcohol to be used
{{{ .7x }}} = liters of alcohol in 70% solution
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I end up with 5 liters, so
(1) {{{ w + x = 5 }}}
and also,
(2) {{{  .7x / 5 = .4 }}} 
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(2) {{{ .7x = 2 }}}
(2) {{{ x = 2.8571 }}}
and, since
(1) {{{ w + x = 5 }}}
(1) {{{ w + 2.8571 = 5 }}}
(1) {{{ w = 2.1429 }}}
2.1429 liters of water must be used