Question 1078685
Let {{{ a }}} = pounds of birdseed needed
Let {{{ b }}} = pounds of sunflower seed needed
--------------------------
(1) {{{ a + b = 50 }}}
(2) {{{ ( .69a + .99b  ) / 50 = .91 }}}
--------------------------------
(2) {{{ .69a + .99b = 45.5 }}}
(2) {{{ 69a + 99b = 4550 }}}
-----------------------------
Multiply both sides of (1) by {{{ 69 }}}
and subtract (1) from (2)
(2) {{{ 69a + 99b = 4550 }}}
(1) {{{ -69a - 69b = -3450 }}}
-----------------------------
{{{ 30b = 1100 }}}
{{{ b = 36.67 }}}
and
(1) {{{ a + 36.67 = 50 }}}
(1) {{{ a = 13.33 }}}
13.33 pounds of birdseed are needed
36.67 pounds of sunflower seed are needed
-----------------------------------------
check:
(2) {{{ ( .69a + .99b  ) / 50 = .91 }}}
(2) {{{ ( .69*13.33 + .99*36.67  ) / 50 = .91 }}}
(2) {{{ ( 9.1977 + 36.3033 ) / 50 = .91 }}}
(2) {{{ 45.501 = .91*50 }}}
(2) {{{ 45.501 = 45.5 }}}
OK