Question 276510
Let {{{a}}} = number of $5 bills
Let {{{b}}} = number of $10 bills
given:
(1) {{{a + b = 15}}}
(2) {{{5a + 10b = 90}}}
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Multiply both sides of (1) by {{{5}}} 
and subtract from (2)
(2) {{{5a + 10b = 90}}}
(1) {{{-5a - 5b = -75}}}
{{{5b = 15}}}
{{{b = 3}}}
and, since
(1) {{{a + b = 15}}
{{{a = 15 - 3}}}
{{{a = 12}}}
You have 12 $5 bills and 3 $10 bills