Question 705999
Let {{{ a }}} = the number of $5 bills he has
Let {{{ b }}} = the number of $10 bills he has
given:
(1) {{{ a + b = 12 }}}
(2) {{{ b = 5a }}}
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Substitute (2) into (1)
(1) {{{ a + b = 12 }}}
(1) {{{ a + 5a = 12 }}}
(1) {{{ 6a = 12 }}}
(1) {{{ a = 2 }}}
and, since
(2) {{{ b = 5a }}}
(2) {{{ b = 5*2 }}}
(2) {{{ b = 10 }}}
The number of $5 bills he has is 2
The number of $10 bills he has is 10
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Notice that this is not really a "money" problem.
You could replace $5 bills with blue marbles
and $10 bills with red marbles, and you have
the same problem.