Hi
Let x represent the pounds of $8 per pound candy
Mixing the candy to create a mixture that cost 9$ per pound
8x + 10.50*4lb = 9*(x + 4lb)
solving for x
8x + 10.50*4lb = 9*x + 9*4lb
1.50*4lb = x
6lb = x, the amount of the pounds of $8 per pound candy
CHECKING our Answer
$8/lb*6lb + $10.50/lb*4lb = $48 + $42 = $90 = 9/lb*10lb