Help with this problem???

Question

Ellen wishes to mix candy worth $2 per pound, x, with candy worth $6 per pound , y, to form 15 pounds of a mixture worth $4.20 per pound. About how many pounds , y, of the more expensive candy should she use?

Answer 1

2x + 6y = 15 x 4.2
x+y=15
y=15-x
2x +6(15-x)=63
2x+90-6x=63
-4x=-27
x=27/4
x=6.75
y=8.25