Question 1016083
.
F=amount of 56%; T=amount of 28%
.
0.56F+0.28T=0.40(T+F)
0.56(9oz)+0.28T=0.40(T+9oz)
5.04oz+0.28T=0.40T+3.6oz 
1.44+0.28T=0.40T
1.44=0.12T
12=T
.
ANSWER: 12 ounces of 28% cocoa should be used.
.
CHECK:
.
0.56F+0.28T=0.40(T+F)
0.56(9oz)+0.28(12oz)=0.40(12oz+9oz)
5.04oz+3.36oz=0.40(21oz)
8.4oz=8.4oz
.