Question 706349
Let {{{ a }}} = mL of 75% solution needed
Let {{{ b }}} = mL of 25% solution needed
given:
(1) {{{ a + b = 1080 }}}
(2) {{{ ( .75a + .25b ) / 1080 = .5 }}} 
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(2) {{{ .75a + .25b = .5*1080 }}}
(2) {{{ .75a + .25b = 540 }}}
(2) {{{ 75a + 25b = 54000 }}}
Multiply both sides of (1) by {{{ 25 }}}
and subtract (1) from (2)
(2) {{{ 75a + 25b = 54000 }}}
(1) {{{ -25a - 25b = -27000 }}}
{{{ 50a = 27000 }}}
{{{ a = 540 }}}
and
(1) {{{ 540 + b = 1080 }}}
(1) {{{ b = 540 }}}
540 mL of 75% acid solution must be used
check:
(2) {{{ ( .75*540 + .25*540 ) / 1080 = .5 }}}
(2) {{{ ( 405 + 135 ) / 1080 = .5 }}}
(2) {{{ 540 = .5*1080 }}}
(2) {{{ 540 = 540 }}}
OK