Answer: 9 5-dollar bills and 4 20-dollar bills.
Explanation:
5x+20y=125
x+y=13
You can isolate one of the variables so you can use substitution. I'll isolate x.
Subtract y from both sides to isolate x.
x=-y+13
Substitute the x in the first equation for -y+13, since they have the same value.
5(-y+13)+20y=125
Use distributive property.
-5y+65+20y=125
Combine like terms.
15y+65=125
Subtract 65 from both sides.
15y=60
Divide 15 from both sides.
y=4
Now, plug in 4 for y in either of the first equations. The second is easier.
x+(4)=13
Subtract 4 from both sides.
x=9