Question 1042372
Let {{{ a }}} = kg of the 10% chocolate needed
Let {{{ b }}} = kg of the 60% chocolate needed
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(1) {{{ a + b = 200 }}}
(2) {{{ ( .1a + .6b ) / 200 = .45 }}}
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(2) {{{ .1a + .6b = .45*200 }}}
(2) {{{ .1a + .6b = 90 }}}
(2) {{{ a + 6b = 900 }}}
Subtract (1) from (2) 
(2) {{{ a + 6b = 900 }}}
(1) {{{ -a - b = -200 }}}
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{{{ 5b = 700 }}}
{{{ b = 140 }}}
and
(1) {{{ a + b = 200 }}}
(1) {{{ a + 140 = 200 }}}
(1) {{{ a = 60 }}}
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60 kg of the 10% chocolate are needed
140 kg of the 60% chocolate are needed
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check:
(2) {{{ ( .1*60 + .6*140 ) / 200 = .45 }}}
(2) {{{ ( 6 + 84 ) / 200 = .45 }}}
(2) {{{ 90 = .45*200 }}}
(2) {{{ 90 = 90 }}}
OK