Question 691502
Let {{{ a }}} = ml of 20% solution needed
Let {{{ b }}} = ml of 60% solution needed
given:
(1) {{{ a + b = 100 }}}
(2) {{{ ( .2a + .6b ) / 100 = .5 }}}
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(2) {{{ .2a + .6b = 50 }}}
(2) {{{ 2a + 6b = 500 }}}
Multiply both sides of (1) by {{{ 2 }}}
and subtract (1) from (2)
(2) {{{ 2a + 6b = 500 }}}
(1) {{{ -2a  - 2b = -200 }}}
{{{ 4b = 300 }}}
{{{ b = 75 }}}
and, since
(1) {{{ a + b = 100 }}}
(1) {{{ a + 75 = 100 }}}
(1) {{{ a = 25 }}}
25 ml of 20% solution is needed
75 ml of 60% solution is needed
check:
(2) {{{ ( .2a + .6b ) / 100 = .5 }}}
(2) {{{ ( .2*25 + .6*75 ) / 100 = .5 }}}
You can finish the check