Question 1067513
Let {{{ a }}} = ounces of 3% cobalt blue color needed
Let {{{ b }}} = ounces of 7% cobalt blue color needed
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(1) {{{ a + b = 256 }}}
(2) {{{ ( .03a + .07b ) / 256 = .05 }}}
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(2) {{{ .03a + .07b = .05*256 }}}
(2) {{{ .03a + .07b = 12.8 }}}  
(2) {{{ 3a + 7b = 1280 }}}
Multiply both sides of (1) by {{{ 3 }}} 
and subtract (1) from (2)
(2) {{{ 3a + 7b = 1280 }}}
(1) {{{ -3a - 3b = -768 }}}
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{{{ 4b = 512 }}}
{{{ b = 128 }}}
and
(1) {{{ a + b = 256 }}}
(1) {{{ a + 128 = 256 }}}
(1) {{{ a = 128 }}}
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128 ounces of 3% cobalt blue color are needed
128 ounces of 7% cobalt blue color are needed
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check:
(2) {{{ ( .03a + .07b ) / 256 = .05 }}}
(2) {{{ ( .03*128 + .07*128 ) / 256 = .05 }}}
(2) {{{ ( 3.84 + 8.96 ) / 256 = .05 }}}
(2) {{{ 12.8 = .05*256 }}}
(2) {{{ 12.8 = 12.8 }}}
OK