Question 1207588
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If the speed of getting the answer is important and formal algebra is not required -- e.g., in a timed competitive math contest -- here is a quick and easy way to solve this kind of problem using logical reasoning and simple mental arithmetic.<br>
If all 48 bills were $10 bills, the total would be $480.
The actual total is $880, which is $400 more than $480.
Each $20 bill is worth $10 more than each $10 bill.  The number of $20 bills needed to make the additional $400 is 400/10 = 40.<br>
ANSWER: 40 $20 bills; 48-40 = 8 $10 bills<br>
NOTE: Even if the problem required a formal algebraic solution, solving problems using logical reasoning and mental arithmetic is excellent brain exercise.<br>