Question 755841
Let {{{ a }}} = liters of 75% solution needed
Let {{{ b }}} = liters of 25% solution needed
{{{ .75a }}} = liters of disinfectant in 75% solution
{{{ .25a }}} = liters of disinfectant in 25% solution
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given:
(1) {{{ a + b = 25 }}}
(2) {{{ ( .75a + .25b ) / 25 = .35 }}}
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(2) {{{ .75a + .25b = .35*25 }}}
(2) {{{ .75a + .25b = 8.75 }}}
(2) {{{ 75a + 25b = 875 }}}
Multiply both sides of (1) by {{{ 25 }}} and
subtract (1) from (2)
(2) {{{ 75a + 25b = 875 }}}
(1) {{{ -25a - 25b = -625 }}}
{{{ 50a = 250 }}}
{{{ a = 5 }}}
and, since
(1) {{{ a + b = 25 }}}
(1) {{{ 5 + b = 25 }}} 
(1) {{{ b = 20 }}}
5 liters of 75% solution are needed
20 liters of 25% solution are needed
check:
(2) {{{ ( .75a + .25b ) / 25 = .35 }}}
(2) {{{ ( .75*5 + .25*20 ) / 25 = .35 }}}
(2) {{{ ( 3.75 + 5 ) / 25 = .35 }}}
(2) {{{ 8.75 = .35*25 }}}
(2) {{{ 8.75 = 8.75 }}}
OK