Question 1181289
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A quick and easy mental solution, if formal algebra is not required....<br>
(1) Imagine having 30 bills that were all $10 bills.  The total would be $300, which is $130 less than the actual total.
(2) Exchanging a $10 bill for a $20 bill keeps the total number of bills the same but increases the total value by $20-$10=$10.
(3) The number of exchanges needed to make up that additional $130 is $130/$10=13.<br>
So 13 $10 bills were exchanged for $20 bills, leaving you with 13 $20 bills and 30-13=17 $10 bills.<br>