Question 855494
Let {{{ a }}} = Bon Jovi's sales in millions
Let {{{ b }}} = Bruce Springsteen's sales in millions 
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(1) {{{ a + b = 415.2 }}}
(2) {{{ b = a - 6.1 }}}
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Substitute (2) into (1)
(1) {{{ a + a - 6.1 = 415.2 }}}
(1) {{{ 2a = 415.2 + 6.1 }}}
(1) {{{ 2a = 421.3 }}}
(1) {{{ a = 210.65 }}}
and
(2) {{{ b = 210.65 - 6.1 }}}
(2) {{{ b = 204.55 }}}
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210.65 = Bon Jovi's sales in millions
204.55 = Bruce Springsteen's sales in millions