Question 941677
Let {{{ a }}} = mass in kg  of box a at beginning
Let {{{ b }}} = mass in kg of box b at beginning
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given:
(1)  {{{ a = b + 3.45 }}} 
(2) {{{ a - 1.2 = 4b }}}
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(1) {{{ a - b = 3.45 }}}
(2) {{{ a - 4b = 1.2 }}}
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Subtract (2) from (1)
(1) {{{ a - b = 3.45 }}}
(2) {{{ -a + 4b = -1.2 }}}
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{{{ 3b = 2.25 }}}
{{{ b = .75 }}}
and
(2) {{{ a - 1.2 = 4*.75 }}}
(2) {{{ a = 1.2 + 3 }}}
(2) {{{ a = 4.2 }}}
Box a was 4.2 kg
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check:
(1) {{{ a - b = 3.45 }}}
(1) {{{ 4.2 - .75 = 3.45 }}}
(1) {{{ 3.45 = 3.45 }}}