Question 1077533
Let {{{ a }}} = liters of 20% solution needed
Let {{{ b }}} = liters of 4% solution needed
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(1) {{{ a + b = 1600 }}}
(2) {{{ ( .2a + .04b ) / 1600 = .1 }}}
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(2) {{{ .2a + .04b = 160 }}}
(2) {{{ 20a + 4b = 16000 }}}
(2) {{{ 5a + b = 4000 }}}
Subtract (1) from (2) 
(2) {{{ 5a + b = 4000 }}}
(1) {{{ -a - b = -1600 }}}
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{{{ 4a = 2400 {{{
{{{ a = 600 }}}
and
(1) {{{ a + b = 1600 }}}
(1) {{{ 600 + b = 1600 }}}
(1) {{{ b = 1000 }}}
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600 liters of 20% solution are needed
1000 liters of 4% solution are needed
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check:
(2) {{{ ( .2a + .04b ) / 1600 = .1 }}}
(2) {{{ ( .2*600 + .04*1000 ) / 1600 = .1 }}}
(2) {{{ ( 120 + 40 ) / 1600 = .1 }}}
(2) {{{ 160 / 1600 = .1 }}}
(2) {{{ 160 = 160 }}}
OK