Question 962369
Let {{{ a }}} = pounds of 79% peanuts needed
Let {{{ b }}} = pounds of 11% peanuts needed 
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(1) {{{ a + b = 7 }}}
(2) {{{ ( .79a + .11b ) / 7 = .49 }}}
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(2) {{{ .79a + .11b = .49*7 }}}
(2) {{{ 79a + 11b = 343 }}}
Multiply both sides of (1) by {{{ 11 }}} and
subtract (1) from (2)
(2) {{{ 79a + 11b = 343 }}}
(1) {{{ -11a - 11b = -77 }}}
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{{{ 68a = 266 }}}
{{{ a = 3.9118 }}}
and
(1) {{{ a + b = 7 }}}
(1) {{{ 3.9118 + b = 7 }}}
(1) {{{ b = 3.0882 }}}
3.9118 pounds of 79% peanuts are needed
3.0882 pounds of 11% peanuts are needed 
check:
(2) {{{ ( .79a + .11b ) / 7 = .49 }}}
(2) {{{ ( .79*3.9118 + .11*3.0882 ) / 7 = .49 }}}
(2) {{{ ( 3.0903 + .3397 ) / 7 = .49 }}}
(2) {{{ 3.43 = .49*7 }}}
(2) {{{ 3.43 = 3.43 }}}
OK
hope I got it