Question 960987
Let {{{ a }}} = liters of 45% solution needed
Let {{{ b }}} = liters of 70% solution needed
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(1) {{{ a + b = 20 }}}
(2) {{{ ( .45a + .7b ) / 20 = .6  }}}
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(2) {{{ .45a + .7b = 12 }}}
(2) {{{ 45a + 70b = 1200 }}}
Multiply both sides of (1) by {{{ 45 }}}
and subtract (1) from (2)
(2) {{{ 45a + 70b = 1200 }}}
(1) {{{ -45a - 45b = -900 }}}
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{{{ 25b = 300 }}}
{{{ b = 12 }}}
and
(1) {{{ a + 12 = 20 }}}
(1) {{{ a = 8 }}}
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8 liters of 45% solution are needed
check:
(2) {{{ ( .45a + .7b ) / 20 = .6  }}}
(2) {{{ ( .45*8 + .7*12 ) / 20 = .6  }}}
You can finish this