Question 1012301
# of 1 dollar bills = X
# of 5 dollar bills = Y
There is a total of 12 bills so: 
X + Y = 12
The total amount of money is 24 dollars so:
Multiply X and Y by their $ values 
1*X + 5*Y = 24
Change X + Y = 12 to X = 12 - Y
Substitute X in 1X + 5Y = 24
To get 12 - Y + 5*Y = 24
Then simplify that equation to get 4*Y = 12
Simplify further to get Y = 3 
X + Y = 12
X + 3 = 12
X = 9
# of 1$ bills = 9
# of 5$ bills = 3