Hi, I've been scratching my head trying to get this to work

the basics are as follows

A1 + B1 = C1 (D1 = C1*20%-B1)


eg

800 + 200 = 1000 ( 0 )



Now I want to change the value in A1 but need a formula for B1 so that (D1 = C1*22%-B1) leaving value (0)