use a table
Cost*amount=value
add collumes and multiply rows
C x A = V
6 y = 6y
9 15-y= 9(15-y)
7 15 = 105
6y+9(15-y)=105
6y+135-9y=105
-3y+135=105
subtract 135
-3y=-30
divide -3
y=10
15-y=15-10=5
10 pounds of $6 mixture
5 pounds of $9 mixture
check
10*6=60
5*9=45
60+45=105
7*15=105
105=105
check
10 pounds $6
5 pounds $9