Hello, Patience,

let x be the amount of the cheaper coffee

let y be the amount of the more expensive coffee

then you get a system of simultanous equations:

A: x + y = 100

B: 9x + 12y = 11.25 * 100

I don't know which method you use to solve this system but you should get the results:

x = 25 and y = 75

EB