Question 782624
Let {{{ a }}} = ml of 2% acid needed
Let {{{ b }}} = ml of 10% acid needed
{{{ .02a }}} = ml of acid in 2% acid
{{{ .1b }}} = ml of acid in 10% acid
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(1) {{{ a + b = 4000 }}}
(2) {{{ ( .02a + .1b ) / 4000 = .04 }}}
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(2) {{{ .02a + .1b = .04*4000 }}}
(2) {{{ .02a + .1b = 160 }}}
(2) {{{ 2a + 10b = 16000 }}}
(2) {{{ a + 5b = 8000 }}}
Subtract (1) from (2) 
(2) {{{ a + 5b = 8000 }}}
(1) {{{ -a - b = -4000 }}}
{{{ 4b = 4000 }}}
{{{ b = 1000 }}}
and, since
(1) {{{ a + b = 4000 }}}
(1) {{{ a = 3000 }}}
3000 ml of 2% acid is needed
1000 ml of 10% acid is needed
check:
(2) {{{ ( .02a + .1b ) / 4000 = .04 }}}
(2) {{{ ( .02*3000 + .1*1000 ) / 4000 = .04 }}}
(2) {{{ ( 60 + 100 ) / 4000 = .04 }}}
(2) {{{ 160 = .04*4000 }}}
(2) {{{ 160 = 160 }}}
OK