So, Using your conversion chart(if you have one) you'd see 1 liter makes 1000 milliliters. So, if you have half of a liter(0.5), then you have 500 milliliters. If he adds 300 more, then you get 800 milliliters total, which is also equal to 0.8 liters. Hope this helps!
513 as a percentage of 900 can be re-written as;

= 57 percent
Let event A = Caroline buys fruit, event B = Caroline buys CD, Ac and Bc are complementary events.
Events AB, ABc, AcB and AcBc are jointly exhaustive and disjoint, hence P(AB) + P(ABc) + P(AcB) +P(AcBc) =1.
Events A and B independent, hence Ac and Bc independent too and probability P(AcBc) = P(Ac)*P(Bc) = (1 - P(A))(1-P(B)) = 0.6*0.4 = 0.24.
Required probability P(AB + ABc + AcB ) = P(AB) + P(ABc) + P(AcB) = 1- P(AcBc) = 1 - 0.24 = 0.76.
Answer: Probability that Caroline buys fruit, a CD or both is 0.76.