Question 66505
x+y=12 x=12-y
5x+10y=100 substitute (12-y) for x in the second equation & solve for y
5(12-y)+10y=100
60-5y+10y=100
5y=100-60
5y=40
y=40/5
y=8 there are 8 10 dollar bills
x+8=12
x=12-8 
x=4 thus 4 5 dollar bills