Hello,

let x be the amount of coffee which costs $9 per lbs

then you have (100-x) lbs of coffee which costs $12

Compare the prices:

9x + (100-x)*12 = 100 * 11.25

9x + 1200 - 12x = 1125

-3x = -75

x = 25

Terefore you have to use 25 lbs of coffee à 9 $/lbs

and 75 lbs of coffee à 12 $/lbs.