Hi,
Reading the problem you can understand that you have 20 currency notes. So, all the currency notes (5 and 10) summed are 20.
Now, you know that there are some 5 and 10 bills of these, but you don't know how many of these are $5 or $10...you only know that summing all these bills give $125.
Setting a sistem of linear equation would be like this:
i call x the $5 bills and y the $10 bills.
x+y = 20
5x + 10y = 125
The final results are x = 15 and y = 5
Hope it is clear!
