Question 926786
Let {{{ a }}} = liters of original solution
Let {{{ b }}} = liters of water to be added
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{{{ .2a }}} = liters of water in original solution
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{{{ 8 }}} L of 59% water = 
{{{ .59*8 = 4.72 }}} L of water
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(1) {{{ a + b = 8 }}}
(2) {{{ .2a + b = 4.72 }}}
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(2) {{{ 20a + 100b = 472 }}}
(2) {{{ 5a + 25b = 118 }}}
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Multiply both sides of (1) by {{{ 5 }}}
and subtract (1) from (2)
(2) {{{ 5a + 25b = 118 }}}
(1) {{{ -5a - 5b = -40 }}}
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{{{ 20b = 78 }}}
{{{ b = 3.9 }}}
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3.9 L of water should be added
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check:
(1) {{{ a + b = 8 }}}
(1) {{{ a = 8 - 3.9 }}}
(1) {{{ a = 4.1 }}}
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(2) {{{ .2a + b = 4.72 }}}
(2) {{{ .2*4.1 + 3.9 = 4.72 }}}
(2) {{{ .82 + 3.9 = 4.72 }}}
(2) {{{ 4.72 = 4.72 }}}
OK