Question 583681
Let {{{ a }}} = pounds of 10% fat chocolate  needed
Let {{{ a }}} = pounds of 60% fat chocolate  needed
given:
{{{ .1a }}} = pounds of fat in 10% chocolate
{{{ .6b }}} = pounds of fat in 60% chocolate
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(1) {{{ a + b = 50 }}}
(2) {{{ ( .1a + .6b ) / 50 = .3 }}}
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(2) {{{ .1a + .6b = .3*50 }}}
(2) {{{ .1a + .6b = 15 }}}
(2) {{{ a + 6b = 150 }}}
Subtract (1) from (2)
(2) {{{ a + 6b = 150 }}}
(1) {{{ -a - b = -50 }}} 
{{{ 5b = 100 }}}
{{{ b = 20 }}}
and, since
(1) {{{ a + b = 50 }}}
(1) {{{ a + 20 = 50 }}}
(1) {{{ a = 30 }}}
30 pounds of 10% fat chocolate is needed
20 pounds of 60% fat chocolate is needed
check:
(2) {{{ ( .1*30 + .6*20 ) / 50 = .3 }}}
(2) {{{ ( 3 + 12 ) / 50 = .3 }}}
(2) {{{ 15/50 = .3 }}}
(2) {{{ 15 = 15 }}}
OK