Question 94746
Let x=number of $5 bills

Then 15-x=number of $10 bills

Now we are told that: (dollars are understood)
5x+10(15-x)=120  get rid of parens

5x+150-10x=120  subtract 150 from both sides

5x+150-150-10x=120-150  collect like terms
-5x=-30 divide both sides by -5

x=6----------------number of $5 bills
15-6=9----------------number of $10 bills

CK

6*5+10*9=120
30+90=120
120=120

Another way:

x=number of $5
y=number of $10

x+y=15--------------------eq1
5x+10y=120-------------------eq2
Notice that if we substituted y=15-x from eq1 into eq2 we end up with the same equation that we initially solved


Hope this helps----------------ptaylor