On the assumption that your answer to (a) is correct, the probability that a fruit is graded as **not** large is $0,7781$, so the probability that a fruit is graded as small is half of this, which is about $0.39405$.
We want the weight $w$ such that the probability that a fruit has weight $\le w$ is $0.39405$. This $w$ is the boundary between small and medium.
Now you have a standard normal distribution problem. If you need further assistance please leave a message.