Question 163455
Let x = cups of pure grapefruit juice added
.
"amt added" + "amt in original" = "desired mix"
x + .08(30) = .10(30+x)
x + 2.4 = 3 + .10x
.90x + 2.4 = 3
.90x = 0.6
x = 0.6/.90
x = 0.6667 
or 
x = 2/3 cups